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Gene Kearns wrote:
On 5 Jun 2006 20:03:26 -0700, wrote:

. Straight weight oils (even as thick as SAE 50) do not have the
?film strength? of multi-viscosity oils such as 5W-20, 5W-30, and
10W-30

This is opposite of what research I have done says and what I have
personally seen in aviation engines. Film strength of multi-viscosity
oils fails when the viscosity improvers fail under extreme pressure.
The oil molecules will re-link in the sump as they cool off, but the
damage to the metallic parts is done.

I would NEVER recommend multi-viscosity oils in extreme pressure
situations (barring really cold starts) and the valve train is a great
place to find such situations. I suspect the article's advice is
driven by people using way to thick an oil.... SAE 50 in an aircraft
engine is the standard... but then it has HUGE clearances compared to
a water cooled boat engine.

One thing about multi viscosity numbers is that in a 10W-30 for
instance, the 10 doesn't correspond to any temperature! It only means
that the oil will flow when cold like a straight 10W will. The 30 on
the other hand, means that the oil will act like a straight 30W at a
given temperature, which is 212F.