A CAUTIONARY NOTE - THIS DOCUMENT WAS PREPARED IN 2002 AND IS NOW (03/2008) OUTDATED - INTENDED TO BE USED AS AN HISTORIC REFERENCE POINT ONLY
Lubricants for the Porsche 928, 928S, 928S4, GT and GTS
The subject of lubricants for the Porsche 928 is both complex yet quite simple
Complex because of the extensive range of modern oils available - a truly bewildering choice
Simple because it is all written in the Porsche Driver’s Handbook - and it has been from new
Many people treat their automotive oil choice as a "religious" issue. They have their own special brands or brews, or they use the brands and types recommended by so-called "experts". Or and sometimes most likely, advertising palys a major role. Sadly, except at the very extremes of racing or rallying where special "brews" or formulations are used there is actually very little difference between the various brands and types of oils - both mineral and synthetic - as long as they meet the API and/or ACEA standards.
No "magic elixir" lubricants(s) exist despite the many claims otherwise. Yes, some oils are marginally better than others and in a general sense you get what you pay for. Ester based Group5 synthetic lubricants ( sometimes using coconut oils etc. and other more advanced substances ) certainly have some benefits. They are quite expensive and the benefits occur mainly under severe heat stress and at temperatures below those experienced in Australia. Some synthetics do minimise wear in valve train components but this only shows up after many hundreds of thousands of kilometres of use - longer than most people keep their cars in most cases
Some synthetics do allow extended drain intervals - but Porsche’s recommended drain intervals are already long enough even with the "normal" recommended mineral oils
As with most things however the dearest lubricant is normally the better lubricant!
No magic additive such as Slick 50 etc. will extend the life of an engine and no research has ever conclusively proven that the likes of Teflon actually work at all in engines ( DuPont says that it doesn’t ) despite the advertising. Some sophisticated chemical additives such as ZDDP and MoS2 however do work very well - at the levels already carefully blended into many high range lubricants hence the higher prices. With these it is not a case of the more the better - in fact it may be the reverse so don’t add anything to your oil - it can do more harm than good. Some may actually pollute exhaust catalysts
Oils ain’t oils!
Sadly, and it is very well known, very few Mechanics and Repairers have a "professional" knowledge of lubricants. It is true too that many prejudices and myths exist in workshops as well along with the ultimate "profit" motive. Most automotive workshop Employees and Employers have never been trained in the lubricants area at all - most sell the product that returns the best profit to them. Or the product they feel "comfortable" with or "believe" in
Therefore and with due respect, do not simply take their word about a product or advice as absolute without first doing some homework for yourself - use the Porsche Handbook too.
And, like at a Doctor’s surgery, always ask questions! You are paying the bill and you own the car!!!
The engine’s oil
NOTE 1 - FIRSTLY, THE 928's ENGINE IS VERY EASY ON ITS ENGINE OIL!
The same lubricant recommendations have applied virtually from first release 928 to final GTS production
Only mandatory improvements in lubricant technology have caused any changes on the way
and many 928 engines already have between 500 000 & 800 000 kms on them without a rebuild!
Oil consumption
The 928 engine is not known to use a lot of oil in normal use. Even engines with considerable total kilometres on them still seem to fit into the original consumption guidelines
Since MY81 the Porsche factory however has consistently published the following consumption rate as the "maximum permissible";
1.5 litres per 1000kms - using the recommended oil type and correct viscosity
Prior to MY81 this figure was 1.0 litre per 700 kms ( approx.)
If your car uses oil at the above rate or less this is judged to be permissible by the Engineers who designed the engine. You might not like it but that’s the way they saw it then
If your car uses more than this you probably have a problem of use or leakage.
The car should then be monitored by using a 15w/20w-50 oil and by keeping accurate records Letting the oil go from Full to Low on the dipstick equals1.5 litres used
Oil pressure
The 928's engine has a relief valve that opens at 9 bar. This is a very high setting!
Revving the engine above 2000 rpm with cold oil IS NOT A GOOD PRACTICE
and should be avoided if possible until a normal hot idle oil pressure is reached. This can take up to 30 minutes
The 928's oil pressure parameters ( with oil temperature at 80C-100C ) are;
At idle 2+
At 3000 rpm 4+ ( later engines 5 )
At 4000 rpm 5 ( later engines )
At 5000 rpm 5
A lighter viscosity and hotter oil may give a low oil pressure reading at idle but flow will be maintained ( and there is no engine load anyway ). Pressure and flow are often in conflict in any engine. Early 32V engines had a modified oil pump in order to show better pressure at idle. The upper rev range oil pressure MUST ALWAYS BE MET on any engine!
From MY82 the cylinder heads contained non-return valves to ensure rapid lubrication of the valve lifters. Earlier cars may suffer from some lifter noise on cold start if the engine has inactive for some time
Oil Filter
Use only a MANN oil filter. These are very special in their construction with a large contaminant capacity and a very high pressure by-pass relief valve. This ensures the earliest possible filtering of cold oil. MANN filters are rated as the very best by many people
Oil cooling
The oil plays a special role in cooling the 928's engine. From the very start the 928 had an oil cooler that is thermostatically controlled. The thermostat commences to open at 87C.
The oil cooler is an integrated part of the 928's overall cooling system and is most important in S4 cars that have "under piston crown" oil jets that operate at 4bar oil pressure and above. These help transfer heat from one of the hottest engine components directly into the sump oil
Engine oil is best kept operating in a temperature "band" from 80C to about 110C and the Porsche’s total cooling system handles this task very well indeed
Oil Specification "Ratings"
Porsche’s 928 oil’s specifications are based on the American Petroleum Institute ( API ) ratings and these will look like this "SH/CD" on the container ( check your Handbook for yours ):
a) API rating examples;
S? is a petrol engine rating with the second letter being the quality stage ( SL> is the latest - in 2004 )
C? is a diesel engine rating with the second letter being the quality stage ( CI-4> is the latest - in 2004 )
The 928 requires both petrol and diesel oil ratings for its engine oil and the container should read SH>/CD> ( see later ). Porsche has always insisted on the secondary diesel oil rating
b) ACEA ratings;
The ACEA ratings were first introduced after the last 928 was built but are important as they are the European Manufacturer’s ratings - and independent of the API
"A" represents petrol engines and "B" applies to diesel engines in the ACEA rating system. The approval quality is shown as a number. The approval year is shown after letter and number ( eg: A3-96 )
If there is also or only an "ACEA" rating on the container it should be "A3-96>/B3-96>" regardless of the API rating!
Note that oil rated as "A1-02" or "B5-01" - as examples - may be unsuitable for the 928!
Viscosity ( thickness )
The correct viscosity for YOUR CAR’S USE and climate is extremely important and it is shown in your Handbook - always use this!
As an example, a 15w-40 viscosity oil is suitable down to -15C ( the 15w bears no relationship to the -15C temperature figure ) which is rarely reached continuously here in Australia. A 15w-40 viscosity oil is suitable for temperatures up to 40C, and at higher temperatures for short periods as encountered in Australia. The 15w-40 multigrade embraces both these extremes
In a general sense;
1 - for 16v engines in Australia a 5w,15w, 20w-50 grade oil is a good all year round choice
If the engine is in good condition and does not consume excessive oil see NOTE 2 later
2 - for 32v engines in Australia a 5w or 15w-50 grade oil is a reasonable all year round choice but a 5w,10w or 15w-40 grade oil will be even better - see NOTE 2 later
3 - for GTS engines in Australia a 0w or 5w-40 grade synthetic oil should be the only choice
Never use an ultra low viscosity oil such as 0w-20 etc in any 928's engine - they may cause premature wear! As well, don’t use a very high viscosity oil unless its needed to meet very high 40C+ continuous temperature needs
Oil type ( Mineral v Synthetic )
All of the early 928s were delivered with a factory fill of mineral oil but the 928 was designed for synthetic oils right from the very start and long before production commenced. The use of these oils was "encouraged" with the introduction of the 32v engine
Since 1992 Porsche have used synthetic oils as a factory fill. Firstly it was Shell in a 5w-40 viscosity and from about 1996, Mobil 1 in a 0w-40 viscosity
There is NO EVIDENCE that synthetic oils will make an engine last longer
There is NO EVIDENCE that modern synthetic oils will make an engine leak, they may in fact assist in controlling minor seepage if the oil is Ester based
There is A LOT OF EVIDENCE that synthetic oils are better at very low and very high temperatures and they will keep the engine’s "internals" cleaner and over a longer period. And as a bonus they will offer a degree of "protection" in an overheated engine and they may reduce valve train wear in certain types of engines
There is A LOT OF EVIDENCE that synthetic gear oils and Auto Transmission Fluid ( ATF ) will make these components last much longer and perform much better at very low and very high temperatures.
Some synthetic gear oils will last for 1 million kms or more and will handle over 125000 gear changes and beyond with very little component wear or lubricant degradation
NOTE 2 - "Mixed fleet" heavy duty ( HDEO ) diesel engine oils
A 928 engine that is in good condition will take very well to a "mixed fleet" HDEO engine oil
Such oils have the previously mentioned API ratings reversed ( instead of say SH/CD it would read CD/SH ) and they usually come as a 10w,15w-40 grade mineral or a 5w-40 grade synthetic
Always check your Handbook to confirm the correct viscosity for your car’s operation
These "mixed fleet" HDEO oils will assist in keeping the engine’s oil cooler, various valving, valve lifters, oil galleries and the oil pump screen cleaner. The latest versions ( 2004> ) will have a CI-4>/SL> rating. As synthetics these oils are at the very top of the oil pecking order!
Earlier API ratings for these HDEO oils - CD>/SH> - will be suitable for your car too - and at a much cheaper price than some oils that are extensively advertised
Oil Change Periods
Porsche has always recommended around 20000kms or a maximum of 12 months ( changed in the spring ) as their oil change period for the 928. They promoted this aspect of the engine’s design. The 928's massive sump capacity, the thermostat controlled oil cooler and the excellent oil filter allowed this with ease and with a hefty safety margin.
The later oils - after 1988 - are much better oils ESPECIALLY THE SYNTHETICS, and they make this oil change period a real breeze! Most CI-4>/SL> HD "mixed fleet" oils would easily go out to two years or 25k of use. The annual time limit however should be observed
and always change your oil in the spring ( as the maximum time interval ) if the kilometre limit has not been reached by then
NOTE 3 - A systematic programme of Used Oil Analysis (UOA) on my car’s synthetic oil over nearly 18 months has confirmed Porsche’s recommendations and with a considerable margin in reserve. This oil would have lasted for around two ( 2 ) years or more with ease and it had a superceded SJ/CF API rating!
As mentioned earlier, always use a genuine MANN oil filter. The design of this filter allows the oil to be filtered earlier when cold thereby extending the life of the oil and the engine
NOTE 4 - Contrary to common perception, changing oil more often than recommended has been shown to increase engine wear. An ongoing University of Michigan study has shown that the greatest wear occurs in the first 3000km of an oil’s life in an engine! There are many reasons for this which we cannot cover here
NOTE 5 - If you race your car a very careful selection of the oil is needed. A reduced interval may be needed too and this can be confirmed by a $50 UOA
Gearbox ( and the auto transmission car’s diff )
The trans-axle requires a 75w-90 GL5 API rated gear oil
A fully synthetic 75w-90 will always do a better job than a mineral oil. Some of these new oils are rated as both GL4 and GL5. The GL5 rating must be present on the oil’s container when used for the 928
Use Porsche’s recommended oil change period if you are using a normal GL5 mineral oil
If you choose to use a synthetic GL5 gear oil you can safely double the oil change period or it can be done after 100000kms use
NOTE 6 - DO NOT ADD ANY ADDITIVES TO MANUAL OR AUTO TRANSMISSIONS!
Auto Transmission
The ATF is the most complex lubricant used in your car and it needs to be regularly checked
as its task is truly amazing. It is a hydraulic oil, a coolant, a cleaner, a friction medium and a sophisticated gear oil too - all in one very sophisticated combination
All modern ATF’s are part synthetic and a GM-Dexron 111 rated ATF is recommended for both the three and four speed 928 transmissions
Use Porsche’s recommended ATF change period if you use a "normal" ATF and changing the ATF, draining the torque converter and changing the filter is really all that is needed
NOTE 7 - There is some evidence that "flushing" any auto transmission may cause premature transmission failure! Avoid flushing your transmission unless some guarantees are given by those recommending the process - the guarantees should be bullet proof!
A fully synthetic ATF such as Castrol’s Transmax Z is a real plus in the 928's. This fluid seems ideally suited to the Mercedes Benz made transmissions and you can expect smoother changes and a reduction in operating temperature of up to 20C as a minimum. The temperature reduction prolongs the life of all components
If you do use Castrol’s Transmax Z the recommended change interval can safely be doubled or it can be done after 100000kms use
Power Steering ( PAS )
The correct lubricant for your power steering is the same ATF as used in the Auto transmission. This is a ATF with Dexron 111 compliance. It should be noted that the PAS’s fluid operates at up to 100bar. It gets quite hot at times operating around the 80-90C range
due to its high operating pressures and its location on the vehicle
The power steering’s ATF should be changed every two years or so
NOTE 8 - If you are using Castrol’s Transmax Z in your Auto you can also use it for the PAS
It is probably best to use a suction pump to withdraw the fluid from the reservoir ( this holds about 300ml ) topping up each time - then running the car for a day or two. Do this 5 or 6 times and you will have most of the old ATF removed and a very slightly diluted "Z" in place
NOTE 9 - For your interest I use the following lubricants in my 1989 928S4 Auto:
Engine - Mobil Delvac 1 5w-40 CI-4/SL - a HDEO mixed fleet synthetic diesel oil
I now do a UOA every six months and the oil is changed annually or at 20k - earlier if the UOA determines this
Auto transmission & PAS - Castrol Transmax Z synthetic Dexron 3 rated ATF
( a "lifetime" fill )
Diff - Castrol Syntrax 75w-90 synthetic GL5 gear oil
( a "lifetime" fill )
If it was a manual 928 then Castrol Syntrax 75w-90 would be in the transaxle