Thanks all, for the info on the gear oil , sounds like the SAE30 is the way to go.

I have been thinking the idea over of rebuilding the transmission next year.
I was considering taking the M41 apart, obtaining new bearings and synchro rings and using whatever parts I can from a less worn M40.

I have a second M41,well the back half of it, the case is snapped off, 9 owner used overdrive in reverse by accident, or so i was told. and the mainshaft is gone, but the od clutch parts may be intact. i only have the one long mainshaft and it is in my car.

the tranny still shifts ok , just getting a little noisy. has anyone else tried this? and will most everything swap over, less the longer mainshaft?
it seems it isn't hard to find good M40's for the parts, but the M41's are harder to find. If all the gears and bearings can be swapped over it might make it easier to finish up with a half decent one without going for broke.

phil