This is paragraph one from the section covering MT90 @ www.redlineoil.com :

RED LINE OIL
Manual Transmission / Manual Transaxle Lubricant
TECHNICAL INFORMATION

RED LINE MTL and MT-90 are designed to provide excellent protection and
improved shiftability for manual transmissions and transaxles. MTL and MT-90
have cured the problem of hard shifting in numerous transmissions which have
notoriously difficult shifting. The secret to their success is two-fold.
First, they have the appropriate coefficient of friction for most manual
transmission synchronizers. Many gear oils, engine oils, and ATFs are too
slippery for proper synchro engagement. Secondly, the viscosity
characteristics of MTL and MT-90 allow proper shifting over the entire
temperature range which the transmission will experience. The synthetic base
oils used have a very high viscosity index which provides relatively
constant viscosity as temperature changes. MTL is a low 70W at very low
temperatures and a high 80W, nearly an 85W, at elevated temperatures,
providing adequate viscosity to prevent wear and deaden gear noise. MT-90 is
a thicker 75W90 version of MTL. The shear stability and oxidation stability
of these products are excellent, thus the physical characteristics of Red
Line MTL and MT-90 will change little with use.

>My point is,
>isn't MT90 a 90 weight gear oil?  Why would we use a heavier single weight
>oil than recommended by the manufacturer?  What am I missing here?
>
>Mick Kalber

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