I'm hoping to find an M41 to replace the auto tranny in my 70 145S. I noticed that IPD has tranny gasket kits. Dunno about difficult of replacement, but I wouldn't think it should be too difficult or require special tools (pullers, presses, etc.) like you would need if you were to be replacing bearings and synchros. I use large sockets as drivers for shaft lip seals typical of tranny input and output shafts. Just go slow and keep an eye out that seal doesn't get too cocked in the housing bore, or get driven in too deep. I'd recommend reassembly with a torque wrench and a drop of medium loctite applied to threads of all bolts (clean male and female threads with carb or electrical contact cleaner and/or compressed air).

Several folks have recommended Redline MTL for the M41. This would have viscosity consistent with the factory recommended "engine oil" but with vastly improved gearbox anti-wear properties. Others have stated that 80W-90 gear oils work fine in high mileage M41's, so you may consider one of the Redline "Shock-proof" gear oils which come in light, medium, and heavy viscosity grades.

Later,
Al