Risse im Zylinderkopf bei Sechszylindermotoren im E 30

[Nick]

Schließlich liebst Du doch Deinen Motor, oder?

Ja, deshalb will ich der beste Öl reinkippen.

Auf der BMW Car Club GB Forum gibt es ein sehr interesantes Thread zum Thema Motoröl. Da kann man Fragen stellen, und Beratung von ein Öl-Fachmann bekommen. Er arbeitet in ein Ölvertreib, kennt sich gut aus, und ist unabhängig von die Ölfirmen.

Er hat mich beraten ein Ester 05W-40 zu benutzen. Ich war skeptisch, und habe auch gedacht ein 05W dunn ist - deshalb habe ich stunden lang Berichte und Meinungen über Motoröl gelesen. Ein 05W bedeutet nicht, dass es wasser-dunn ist, es ist einfach nicht zu dick beim kaltstart.
Dann habe ich eine Zusammenfassung geschrieben und an der Öl-Expert geschickt. Kopie von meiner Text und seiner Antwort, leider auf Englisch:-

"A lot of BMW experts say that 0W-xx and 5W-xx oils are too 'watery' and therefore bad, and that an old E30 M20 engine needs at least 15-xx. But I'm starting to think they have misunderstood the viscosity ratings. This is what I think is really going on:-
The ideal viscosity for the engine is the one a correct oil reaches once the engine is at operating temperature - say about 100C, and this is best described by the second part of a multigrade designation. A xxW-40 oil will be at about 12-15 cSt at this temperature, and we can assume this is roughly what the BMW designers had in mind for a M20.
The first part of the designation is the cold behaviour of the oil, and by cold I am beginning to think we are talking about a non-warmed up engine ie anything less than 100C, whether that is a 10C spring morning or a -20C winter morning.
As all oils unfortunately become thicker as they get colder, in any non-warmed up engine the oil will be thicker than the optimum 12-15 cSt. This reduces oil flow and increases friction, both of which are bad for the engine. It will also lead to heat build-up, which is believed to be the main reason for cracked cylinder heads on M20s.
Therefore the best oil for an engine is one that has optimum viscosity at high temperatures (ie xxW/40 for an M20), but that doesn’t thicken up too much for my sunny spring morning trips. As a 5W-xx can still manage halfway decent flows at lower temperatures than a 15W-xx, a 5W-xx will be less overthickened at 10C than a 15W-xx. Overall there are no conditions in which a 05W-40 will be thinner than an equal quality 15W-40, but the 05W-40 will be closer to optimum at engine start-up.
Looking at the Motul data sheets, 300V Competition 15W-50 is 18cSt at 100C, and 128cSt at 40C. At 100C it is therefore slightly over optimum, but as a full race engine will run warmer than a road car this is reasonable. But at 40C it is ten times thicker than optimum, and it will be even worse at 10C.
8100 X-cess 5W-40 is 14cSt at 100C, and 83cSt at 40C. At 100C it is in the optimum range. At 40C it is only six times thicker than optimum, and so should also be better at 10C. Therefore the 8100 X-cess 5W-40 is the right oil for a summer only, weekend use M20 engine.
Simon, this leaves me with some questions:
1 - Have I understood correctly ??
2 - If I am right, wouldn’t the 8100 E-tech 0W-40 be even better ??
3 - If I am right, why does 10W-xx and 15W-xx exist any more, they offer no advantages over a 0W-xx ??"




"Yep, you've pretty much got it right. The best "all year round" oil is indeed 5w-40 for the BMW and of course you could use 0w-40 as well.
You need an oil that adequately protects when the engine is hot but also offers good flow when cold as this is where 90% of the engine wear occurs.
10w-40 is also a grade that can be used but it's not quite so good on start-up as it doesn't circulate so quickly.
Many people get hung up over thicker oils being better, it's not really the case with most modern engines although there are always exceptions due to design requirement.
The market has changed, mainly driven by the OEM's who have seen the light over lower viscosity oils and the advantages they give related to fuel economy, bhp and emissions.
In many cases a good quality low viscosity synthetic oil is in fact tougher than a high viscosity petroleum oil and numbers like the HTHS and NOACK % if you can get hold of them prove this fact.
The world has changed and oil chemistry has come on leaps and bounds, hopefully engines will last longer because of it.
Cheers
Simon"