Keeping it simple.. for the 996 I would stick to 0w40, 5w40, 5w50 or if you are really frequent with your oil changes, 10w40
Keeping it less simple... For the 996 and Boxster, one of the problems with the IMS bearing failures is that it is a sealed bearing from the factory, the grease in the bearing, when hot is very thin (monograde), and as the bearing ages, the seal shrinks slightly, so the grease runs out... However, the oil in that area is not plentiful enough so it does not get into the bearing to replace it... Now the interesting thing is, in theory, to avoid IMS bearing failure, with a standard bearing, you would be better off with a lower viscousity oil which is more likely to make it past the clearances between the bearing and its seal.. However, with a modified or improved bearing with the seal removed, the bearing would get plenty of oil and not fail.
A 944 with a 10w60 oil will not be damaged, yes, a 60 grade oil is more than a 5th thicker than a 50 grade, but it will not be damaged by the use of that oil.
The paradox is, I have problems finding an oil company I trust who put enough ZDDP into an oil to satisfy the 944 top end, however, a 15w50 with above 1100ppm of ZDDP would be ideal, but since the dissapearance of Mobil-1 Motorsport 15w50 nothing I have tried and used long term has satisfied me as being ideal.
JZMachtech, from memory, have been using Castrol Magnatech 10w60 for about 10 years (not sure if they still are) and I have never seen a problem of a 944 serviced by them (even though they stopped very recently) that had evidence of oil starvation, oil flow or oil pressure related issues on cars serviced there.
However, I have seen a correlation, a clear one at that, of 944's and 993's which are internally dirty during a rebuild, which is a stark contrast to the cleaner internals of a car run for years on Mobil-1 Motorsport, but I think that has much more to do with their detergent package rather than viscosity.
Yes Opie sell a lot of oil, but when it comes down to 944, 968 and 911 air cooled engines, trust me, 10w60 Mobil is much better than a thinner oil which may not have the ZDDP required to protect high load areas, in particular, the cam and followers in a 924, 944, 968 or 928 engine.
Having said that though... I am sure a lot of 944's have been running on all kinds of cheap muck supermarket oil for years, and many of them have probably gone through my workshop, yet I do not know when I last had to change lifters or camshafts in a 944 due to wear.. so.. fill your boots up, pay your money and make your choice!
However... thinking some more, there may be a correlation between low ZDDP levels in oil and cam sprocket wear due to the sheering forces between the chain and the sprockets, as well as within the links of the chain on 944S, 944S2, 968 and 928S4 models.. But I have never read any studies into the benefits of ZDDP in cam chains and sprockets... However, it is a problem which can be avoided with routine maintenance, but may be an issue which would explain why some 944 16v and 928 32v engines suffer with hooked sprockets more than others, other than the usual blame being with infrequent chain replacement.
Recently I depleated my last stocks of Mobil 1 motorsport, which was an upgrade choice for our customers over other mobil grades, and am still choosing if we should offer the Longlife 10w60 mobil-1 product.. However, due to my experience of mobil products, it seems to be the top contender.
So this whole subject has some interest for me.
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