I thirdly recommend Redline MTL. AFAIK Redline MTL has been used for around 70K miles on my tranny. I've put 30K on it so far and replace it every 10K miles although the used fluid looks almost the same as the new. My tranny doesn't make any noise, dead quiet. Only issue I have is it's a little difficult to put it in 1st sometimes... Oh and I always get 2 bottles in there.
--
'88 245,M47,Virgos,235K+

Don't use regular gear oil. ATF is specified for the sake of the synchros which rely on friction characteristics of the lubricant used. With regular gear oil I would expect the box to act as if it had no synchros at all.

Red Line MTL works fine, and overfill the box by a pint if you can.

If memory serves me correctly, the torque rating of the M47 is around 250 foot-pounds - certainly not any higher!

I don't think it's a "weak" tranny per se, depending on the application. Mine had 300K on it before it needed a rebuild, and it was functioning fine, just the bearings for fifth gear were screaming. I changed to Redline MTL, a synthetic brand of fluid that is pretty thin and is approved for use in the M47 when I got the car at about 200K miles. I think it helped with the tranny's longevity. It certainly quieted it down some and made the shifts smoother. I also over-filled it to give the fifth gear bearings a little more lubrication. They used these trannys for the turbo models, so I guess they can handle some higher power. I wouldn't use gear oil in it only because it's specifically not recommended by the manufacturer. I'm thinking they probably have a good reason

My limited knowledge of the M47 is that is does indeed call for ATF instead of the traditional gear lube to be used. It also seems to benefit from "overfilling" by about 1/2 quart. This is acheived by jacking the car up so that the fill plug is the highest it can be in relation to the rest of the tranny, and filling until the fluid runs out of the fill hole. I would suggest that a fully synthetic, high quality lube be used. (i.e Red Line, Royal Purple, Amsoil, etc.). There may very well be more information on the search function as well.