Warren describes it well. Although the oil you recommend is probably the best for the M46/OD, the proper thing to do is replace the OD piston orings and the rest of the orings while you are at it. Changing to a higher viscosity oil (more pressure) may encourage OD clutch slipping, since it is not type F. The OD clutch, or material, is not readily available according to Duane. My wife's car ate the OD clutch when one of the pressure regulator springs was broken. The low pressure to the engaging pistons reduced the force on the OD clutch, allowing it to slip under load. She didn't notice.