[TR] Proper oil for TR4 Gearbox

[pbrandsema at triad.rr.com]

I'm currently using heavy hypoid oil in my TR4 gearbox, which is fitted with overdrive.  I've read that a lighter oil should be used, that the syncromesh gears don't like the heavy stuff.  However, I've been running this oil for a number of years, and the gearbox shows no signs of problems.  Is this a case of "if it's not broke, don't fix it"?  What are recommendations of proper viscosity.  I don't have the workshop manual in front of me, but I'll bet it calls for the heavy oil.

Thanks,
Paul