A few more comments on oil in general, just to stir the pot:
A lot of people seem to believe the marketing hype that 20w50 is the
ultimate "heavy-duty" motor oil for street use. The fact is, multi-grade
oils start with the thinner, lighter stock (in this case 20wt), and add
viscosity improvers. Which is fine, until at high RPM, or high load stresses
in the bearings, these molecules (styrene isoprene) break down, and you're
effectively running 20wt. Unless you live in a region with widely varying
temperatures year-round (like the high desert), 20w50 is basically just a
crutch for not changing your oil summer and winter. But since the viscosity
improvers break down over time, it's not like it's saving you on oil changes
overall. I'm not saying there is a problem under normal use, but if you only
drive your MG in the warmer months, and then you drive it hard, you are
probably better off with 30wt.
Before running a really light oil like 10w30 (in winter, for example), check
the packaging for a sunburst logo reading "American Petroleum Institute
Certified -- For Gasoline Engines". This indicates an oil with an additive
package which reduces the amount of anti-scuff/anti-wear additive (ZDP) in
favor of extra friction reducers. Basically, this is a political payoff for
automakers -- it is intended to increase gas mileage by some tiny increment
in new cars, at the expense of offering significantly less protection for
high performance engines, particularly older designs.
As for oil used in transmissions, well, there are no combustion byproducts
in transmissions, so detergents just plain aren't necessary (though I
wouldn't go so far as to say they will hurt). Also, oil pressure is not a
consideration, since lubrication is by splashing. When you consider that the
viscosity improvers in multi-grade oil have no effect at lower temperatures
-- the molecules actually unfold as the temperature goes up, which is what
stiffens the viscosity -- and that the transmission operates at a much lower
temperature than the engine, it's hard to avoid the conclusion that a
single-weight non-detergent oil is actually what is called for. In light of
this, the factory recommendation to use whatever is specified for the engine
seems like laziness, or more likely, an over-simplification that makes no
real-world difference. If it works with crushed bananas, then the difference
between 20w50 and 30wt is probably moot.
In the case of overdrive transmissions, however, the oil does serve a
hydraulic function, so viscosity may very well have an effect. I would
wonder if 20w50 (or 10w40), at lower temperatures, might be too thin to
provide adequate pressure, thus causing overdrive actuation problems, though
I can't recall hearing of such a case. But it is clear that 90wt gear oil is
problematic. And crushed bananas are definitely not recommended...
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires
|