Some interesting info:


Without getting technical, I can tell when it's time to switch from 10W30
M1 to 15W50 M1 by my 928GT's pressure gauge. I use a 50/50 blend of 10W30 and 15W50 M1 during the winter. The gauge at idle remains above 2 Bar.
However, when ambient temps get above 30C the gauge will read 1.5 Bar at idle. In Summer, with straight 15W50 M1, the gauge reads 2 Bar (or more) at idle.

Remember, even though ambient temps may vary considerably, the 928's normal upper operating temp is about 100C for the coolant. The oil used should have the viscosity needed to handle those high engine temps, regardless of the ambient temp. YMMV.

~ Merry motoring ~  Ed Ruiz


Hi Tim,
the most important things to consider are these;
1 - the oil must have a minimum HTHS viscosity of 3.5 cSt (at 150C) and preferably about 4 to 4.5. These will not normally be achieved by a 0w-30, 10w-30 or almost any oil under a 40 at the upper range (eg 0w-40,5w-40)
2 - A HTHS visc. above 4.5 cSt saps power (and fuel) for little effect
3 - 15w-40 mineral HDEOs are excellent for a well maintained 928 (see 3 below) and their 5w-40 synthetic brothers are even better
4 - oil pressure at idle is not that important - 1.5bar> is ample as flow is always maintained (constant displacement oil pump). Obtaining 5bar at 4000-5000k rpm is most important
5 - Follow the Handbook's API specification SL/CH>(or ACEA A3/B3) and viscosity requirements for your "normal" ambient range
6 - All oils are not the same - one 15w-50 can be at the low end of the 15w range and the high end of the 50 range - another may be reversed etc
7 - The 928's oil temperature is closely controlled with the oil cooler thermostat opening at 87C and likely to be fully open near 95C
8 - Engine Oil needs to be above about 55c to activate anti-wear additives and the ideal operating temperature range is about 88-95C
9 - When monitoring my engine's temps I have not seen anything above 100C for the engine oil in normal use (Ambient from -5C to 43C and very long distances above 160km/h) 10 - A 5w-40 oil synthetic is probably the best lubricant for the 928 and is as recommended by Porsche. My own testing confirms this
11 - Heavier oils than really needed (say 20w-50) will tend to by pass the oil filter longer when in the warm up cycle
12 - Synthetic oils have more linear flow characteristics than mineral oils. It is also extremely difficult to accurately plot oils at various temperatures unless the Viscosity Index is known along with the viscosity at both 40C and 100C. Even then the chemistry used can have a significant effect

Check out the previous seven "Oil Condition Reports" from my S4 on the 928 Forum. The eight will be posted in September. Four tests were done on a 15w-50 Shell Helix Ultra synthetic and three (to date) on a 5w-40 HDEO synthetic, Delvac 1.
No significant wear or contaminant variances have occurred over the 32k kms covered in the tests to date

Read this for more information;

Check under "Lubricants for the 928"


Your points are well taken, but I think there are a few minor corrections needed.

First, oil pressure is important as long as the engine is running, including idle. Since the oil pump is turning slowest when the engine is at idle, the pressure will also be the lowest. Personally, I prefer to have at least 1.5 Bar as a minimum pressure, YMMV. So far, at 2k RPM and above, the pressure is pegged at 5 Bar, regardless of the oil's weight.

Second, the only time the oil temp is the same as the coolant temp is upon initial cold startup. From that point on, the oil temps quickly rise to temps approaching those in the combustion chamber (perhaps as high as 250C). Remember, oil lubricates and cools moving parts in the engine (including valves and rings, which are in direct contact or just passed through the combustion chamber), and coolant cools the block and the oil.
If the oil were the same temp as the coolant, it wouldn't need to go through a heat exchanger in the radiator or it's own separate oil cooler.

~ Merry motoring ~ Ed


 Ed Ruiz noted his experience with Mobil 1 and different viscosity oils.
I have noted the following in my '88 S4. (~76K miles, driven on both street and DE's at various tracks).

At normal operating temperature and ambient air temp at ~75 F, the oil pressure at idle, (automatic transmission in N) is:
Mobil dino oil - 10W40 = ~2.5 bar
Mobil One 15W50 = ~3.0 bar
Amsoil Racing Oil 20W50 = ~3.5 bar

When driving on the highway with the engine at full operating temperature, (after 50+ miles @~65 mph), and ambient temps at ~75 F, I have full 5 bar of oil pressure at the following RPM's:
Mobil dino oil - 10W40 = ~ 2250
Mobil One 15W50 = ~ 1800
Amsoil Racing Oil, 20W50 = ~ 1450

Just as info for anyone who is interested.

Gary Knox
West Chester, PA

Ed - the design oil pressure minimum at idle is as per the oil light trigger - about .65bar. An idle pressure of 1.5bar (22lbs is sufficient for an "unloaded" engine. Most 928s show 2 to 2.5 when using the recommended viscosity. The oil's flow is always critical!

The oil's temperature is very slow to rise in relationship to the coolant.
Even after 12 minutes or so at an ambient around 15C the oil is about 8-10C below the core coolant temperature.
In most vehicles without an oil cooler the oil normally runs at about 15-20C warmer than the coolant does.
The 928 is indeed quite different. The oil cooler "washes" much heat from the sump's residual oil temperature quite quickly and typically the cooler is quite intermittent in operation. The oil cooler may indeed remain closed (or rarely open) during much normal road use of the vehicle in ambient temperatures below about 10C - and perhaps even higher!

At individual points within the engine the oil may indeed be quite hot but it is the "core" bulk oil temperature that tells the story. Most heat is added to the oil from the piston & undercrown with the next highest contributor being the various engine bearings. The increase can be linear to the increase in engine speed and load. It is hardly affected at all by an increase in ambient temp.

It is rare for most engines without an oil cooler to post continuous oil temperatures above about 110C for long periods. The critical HTHS viscosity - critical for engine bearing preservation - is measured at 150C for this reason. Most synthetic oils will tolerate sump oil temps around 130C for extended periods without serious and quick deterioration.
(The average oil temps in my 500hp OTR 12ltr Detroit diesels is 103C with a warning at 115C and progressive shutdown at 121C)

The 928 is very easy on its oil because of the large sump capacity, good filtration, and the thermostatic oil cooler which tends to keep the oil in the narrow "ideal" operating band from about 85C-95C and rarely raises it to 110C according to my 100 odd testing sequences. This is why Porsche have always recommended long oil drain cycles which have been confirmed by my professional Oil Analysis testing regime. This has shown that the oil has an "easy ride" with no evidence of oxidation or viscosity gain caused by high oil operating temperatures! And no abnormal wear at any stage!

Glen - The 928's engine has had two different relief valve settings 8 and
9 bar if I recall correctly. This is VERY HIGH pressure by any automotive engine standards

The opening of the oil cooler by-pass is about 0.5bar I think this is to prevent damage even with very hot oil

The opening of the oil filter by-pass is to enable direct oil flow around the engine (or dumped back to the pump via the relief valve) when the oil is cold and the pressure near the relief valve setting. This by-passes the oil filtering media and prevents damage to it or other components.
Typically a thicker mineral oil (say 15w-50) would be in full or partial by pass for longer than a thinner synthetic (say 5w-40) MANN (and possibly Mahle) filters have a high relief valve "crack" point to ensure that oil begins being filtered quite early in the warm up cycle as the oil becomes less viscous

It is likely that the 928's oil is fully circulated about once per 20-25sec or so at 4000rpm. Not having the flow capacity I cannot confirm the actual figure but it would be in this vicinity I believe

The oil pump's pressure is controlled via the relief valve as mentioned earlier. The pump's flow is fixed (per revolution) due to its constant displacement design The 928's engine has generous oil galleries and an excellent hydraulic valve lifter design that enables the high basic normal hot high pressure of 5bar+ to be sustained

I hope this is of interest



Über die API-Klassen:

American Petroleum Institute
Klassifizierung aufgrund der Betriebsbedingungen der Motoren, getrennt nach Otto- und Dieselmotoren.
Damit bestimmen die Motorenhersteller motorenseitig und die Ölgesellschaften schmierstoffseitig die
erforderliche bzw. die gebotene Klasse.
Die Bezeichnung API ist auf jedem Motorenölgebinde zu finden.
Der erste Buchstabe ist immer ein S oder C.
API S - Steht für Ottomotoren (Service-Klassen auch Spark Ignition)
API C - Steht für Dieselmotoren (Commercial-Klassen auch Compression Igniton)
Der zweite Buchstabe steht für die Klasse (Qualität). Je höher der Buchstabe im Alphabet,
je besser die Qualität. Die derzeit höchsten API Spezifikationen sind SJ / CE
Die Klassen sind wie folgt Definiert:

Ottomotorenöle API-Klassifikation
SA - Regular-Motoröle evtl. mit Pourdepressant und/oder Antischaummittel
SB - Motoröl für niedrig beanspruchte Otto-Motoren mit Wirkstoffen gegen Alterung, Korrosion und Verschleiss
SC - Motoröl für mittelbelastete Otto-Motoren wie SB plus Wirkstoffen gegen Verkokung
SD - Motoröl für schwere Betriebsbedingungen bei Otto-Motoren (von 1968 bis 1971)
SE - Motoröl für sehr hohe Anforderungen bei Otto-Motoren (von 1971 bis 1972)
SF - Motoröl für sehr hohe Anforderungen bei Otto-Motoren wie SE + verbessertem Veschleissschutz und Schlammtragevermögen
SG - Motoröl für höchste Anforderungen wie SF + Schutz gegen (Schwarz-)Schlammbildung
SH - Motoröl für höchste Anforderungen (ab 1993) wie SG + zusätzlich Anforderungen HTHS und Verdampfungsverlust
(HTHS = High Temperatur High Shear / Hochtemperaturviskosität)