I've been using Millers Oils in my engines and gearboxes for a few years now, with outstanding results. It's not cheap, but it works. Use to use Mobil 1 Delvac (997 Cup car gear oil) in my 997 gearbox, would get too hot and cause shifting issues....switched to Millers and immediately problems went away, and could use in the hottest summer conditions with no issues. Since then, never looked back.
Has Nano-tech, Indycar/motorsport used, etc. etc., it's the real deal.
For the M3, I would use either Millers Oils NT 75W140 LS, or Motul Gear Competition 75W140. Both handle heat extremely well, and have just a little bit of LS additive to let the LSD work. I'm sure the Castrol is perfectly fine too, it always seem to work in my E46 M3 (non FM booster version). I'm sure I'll try the non-LS though....without a LSD oil cooler, temps get hot, and LS additive just makes things more slippery. For example, Miller's Oils will advise track users to use the NON-LS version for LSD/transaxles for this same reason.
I've been using Millers NT 75W110 (non LS) in my 987 transaxle (tranny and LSD all in one case) with an OS Giken with good results. The LSD locks REALLY well. For a transaxle, it's a compromise as the gearbox works best with 75W90 but the LSD really needs 75W140 (impossible to run Giken 80W250 oil in Porsche transaxle), so have been playing with weights and LS/non-LS to see what I like the best. Going back to 75W140 LS next to see if it smooths out the locking action a bit more. Late in sessions, the gear oil gets super hot, and seems to cause the LSD to lock harder....987's didn't come with factory tranny cooler. So the thicker oil should hold up better, and just have to make sure to warm up the gearbox before hammering down.
For 997 GT3, you could get away with 75W90 because it had a true motorsport cooler for the tranny.
For the M3, no compromise is needed, just run what works best for the intended LSD. Giken, definitely use their oil....for all other LSD's, I would probably run 75W140 for track work.
Gear oil is definitely another tool in your box for tuning....
