>LL's recent post advised against the use of 'thinner' oils than 20/50.
Yep, although a case can be made for using an excellent synthetic 10W30 or 10W40, whose film strength (that which keeps metal away from metal in bearings) is at least as high as that of a non-synthetic 20W50, but whose cold viscosity is much lower (so it flows better at start) and whose hot viscosity is also lower (so it consumes less power in viscous drag).
However, I take the conservative position that Porsche wanted lots of protection for our engines, and that a contemporary synthetic 15W50 or 20W50 oil has much higher film strength than any non-synthetic 20W50, flows much better at low temperatures and substantially better at high temperatures, and offers less viscous drag. I found an immediately feelable difference in my engine going from non-synthetic 20W50 to a good (Amsoil Series 2000) 20W50 some years ago, and have stuck with the latter since then.
For what it's worth, Mobil take a similar position (http://www.mobil.com/oneracing/about/products/engineoil/15w_50/index.html)
in recommending the 15W50 version of Mobil 1 sold in the US over the 10W30 version for "high-performance and high-revving cars".
>The owner's handbook for my '79 shark does, indeed, recommend
>20W/50 or 10W/50, but there weren't many other multigrades around
>20 years ago.
Not so; 10W30 was the most common multigrade product; it was just inadequate to the loads and speeds inside a 928 engine used hard.
>I would not for one moment question LL's advice
That's literally too flattering.
>Porsche currently recommend Mobil 1.
Yes, but see above.
>Does LL consider a good 20/50 better than Mobil 1?
In a vigorously driven 928, I'd rather see you use synthetic oil than non-synthetic, and I'd rather see you use 15W50 or 20W50 than 10W30 or even 10W40.
Leonard Laub (firstname.lastname@example.org)
'87 928 S4 stick (black and ever so slightly enhanced)
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