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I believe you answered your own question when you added the phrase "...except for the OD unit," to the second sentence in your post. (You're aware that in the case of the M41, the gearbox and overdrive units use a common lube supply, yes?) The overdrive unit's lube requirements are more like an automatic transmission than a 4-speed, due mainly to the fact it has a lube pump in it.
The shop manuals typically list 30w or 10w40 as an "alternate" oil for the M40, so I would say you're okay using the lighter lube. Might even get you a couple of 10ths mpg increase?
Okay, hopefully I answered your gearbox question satisfactorily, now you can go to the 140/160 area and answer *my* M40 question. :)
Gary L.
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1971 142E ITB racer, 1973 1800ES, 2002 S60 T5
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