Chris,
Just to make sure you're clear on what's available:
75W90
75W90NS
ShockProof Lightweight
ShockProof Heavyweight
are all separate products and then there is also Red Line's Friction Modifier (FM) for LSDs. (Nothing says you can't mix them though, indeed one older Red Line recommendation was for 3 parts 75W90 and 1 part Lightweight Gear Oil. Lightweight Gear Oil is yet another product, not to be confused with Lightweight ShockProof Gear Oil. It can be confusing. Trust me, I found myself confused on a number of occasions: Now wait a minute, was NS for synchros or for N-on S-ynchros??? It is actually recommended for synchromesh transmissions
The bottom line is that think that PHII's recommendation, as reported above by KalunD, is spot on in a GT40 with any kind of built SBF. If you were running a stock SBF and using the car for occasionally bopping around town in style, then you could use 75W90NS (it's doubtful this kind of car would have an LSD, but if it did, then I'd add just enough FM to quiet the LSD chatter.) FM really shouldn't be needed with 75W90, Lightweight ShockProof or Heavyweight ShockProof, but it is the ace in the hole if you should experience LSD chatter, but, again, the rule of thumb is "only enough FM to do the job." Luckily for us, getting gear lube into/out of the tranny is fairly easy; for our poor Porsche brethren, it is a bit of a PITA.
So, once you've decided which gear oil to use, how much do you put in?? It seems that best way to get it at the right level, is to pull (or loosen) one of the output flanges and fill until the oil leaks out of the opening. (retorque to 19-22 ft.lbs. for '72+) This will get enough oil in the gearbox to adequately oil the diff gears/clutches (the ring gear will dip into the oil and spray the pinion at much lower levels.)
I really think that if you are running much HP, you should seriously consider a cooler with the return directed at the diff (a cooler alone will help though.) If you follow the KISS principle, this can be done fairly easily and for a reasonable price and is a cheap insurance policy. You can also spend a lot of money and have a very elegant oiling system in your gearbox. If you need maximum performance and you do this, you can reduce the oil level to reduce the drag it causes. (For most of us, the gain would be negligible.) I don't think I've said anything new here, but I've tried to get a broad discussion in one spot so the newbee, if he doesn't read anything else, will have the basics.
Regards,
Lynn
PS: My daughter tells me that I turn a simple answer into a class lecture - who me??? Nahhhhhhh.