> I assume you (Randall) are using Valvoline SynPower, which
> seems to be their synthetic gear oil.
That's right.
> One thing I had noticed about some of
> the choices was a wider viscosity rating, 75-90 instead of 80-90, and
> in some cases numbers more like 70-140 (or whatever, which I really
> don't remember). I remember thinking that the more liberal numbers
> might be optimistic, perhaps the product of a marketing department,
> or that that oil might not age so well.
There may be something to be said for that, but IMO it's not fair to compare
conventional and synthetic oils. Synthetic is more stable over temperature,
meaning it's viscosity naturally changes less, even without the "viscosity
improver" additives that make conventional oil "multi-viscosity".
Contrary to popular opinion "multi-viscosity" oils do NOT get thicker as they
get hot, they just don't thin out as much as "single weight" oils do.
But the main reason I like full synthetics is the higher film strength. Oil is
all about maintaining a thin film between rubbing metal parts (like gear teeth
and piston rings), so a stronger film just has to be better, IMO. In theory, if
there was always a film between the parts, the parts would never wear out. They
do wear out, obviously, hence the film breaks down under some circumstances. A
stronger film should break down less often ...
I ran "straight-weight" oil in my TR for many years, because I had trouble with
the viscosity improver breaking down and leaving the oil as thin as water.
(Conventional 10W40 oil is basically about 5 weight oil, plus an additive very
similar to STP.) But the full synthetic comes out almost exactly the same
viscosity as it went in, even after 10k miles. The inside of the engine is also
a whole lot cleaner after running synthetic than it ever was running "dinosaur
juice".
Randall
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