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Just my $.02...
I have NEVER, and been told many times to NEVER run gear oil in the OD units. I would also tend to stay away from synthetics or slippery additives. A friend of mine once put the tranny equivlant of slick50 in his overdrive gearbox...and the OD never worked again. My experience has been to stick with dinosaurs.
My choice...I run straight, non-detergent 40wt engine oil. A little thicker than the recommended 30wt, but seems to work fine. Admittedly, my OD on my M41 in my 1800 quit working recently, but the whole unit was found in the trunk of a parts car, and its condition and mileage were and are still unknown. I expect it wasn't very good when I started though, as the sincros were bad when I installed it. The overdrive still worked for more than 150K, and the tranny itself is still quiet.
Like others, I've not had too many problems with excessive gearbox wear on the M40 or M41... They seem to hold up longer than the M400 and M410, so I wouldn't worry too much about running the lighter oil. I expect that the tranny will still outlast the overdrive. When the overdrive does finally quit working for good, then go with gear oil until you do a rebuild.
-Matt
65 1800s
71 145s
72 164e
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