Well, the OD required 30wt oil, which is the driving force behiind specifying it on the M41. I'm not sure about difference internally, I took my PV's M40 apart to scavenge syncro cones to use in repairing its new M41 and didn't notice any real difference in bearing types. Although in typical Volvo fashion, many of the parts that looked alike were actually subtly different (oy, after hours of trying to reassemble a mix-n-match of the best looking parts from each trans I checked part numbers, and used calipers and found they were subtly different, the only parts that actually interchanged were, thankfully, the syncro cones).

Gear oil is typically a little more clingy. From what I understand, the viscosity is measured at a different temperature for gear oil standards, and 90wt gear oil isn't that much different than 30wt motor oil. But I guess the additives are different, gear oil is cheaper because it doesn't have all the additives required to make it more stable in the more dynamic and dirty environment of an engine.

Really, I don't think you'd notice much difference at all. But no advantage to using 30WT.
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I'm JohnMc, and I approved this message.