i am not a gearbox expert. I have dealt with plate LSD's 3 times in my life, i'll list my 3 experiences below.
Fitted an SR20DET to a fwd nissan sunny, bought a box out of an ex rally almera with cusco plate diff. built car up, filled gearbox with whatever was recommended for the standard box. I had never been in a car with a clutch diff but had read they were noisy. when i went for a drive i couldn't believe how noisy it was, and the way the steering would tigten up as i was turning a corner, make a cracking noise, go loose and then start tightening up again....felt like it was going to snap a drive shaft... deffinately not right. i dont know why i did, but i tried a synthetic oil from the local motor factors and the difference was immense, no more loud clunking, no more feeling like i was going to snap a shaft....could still feel the steering pulling straight when applying throttle on a corner but was smooth and only needed a constant force to keep to my intended line on a corner. a light chatter noise on some slow slight turns but more in line with the level of noise i expected before i fitted it IMO the lsd was now working as it should.
Experience 2. went to look at a starlet glanza turbo for my girlfriend, saw a tidy one with lots of nice mods so took it for a drive. it immediately became aparent that this car had a clutch lsd fitted (fresh import from japan) i had the same feeling of the steering pulling straight (but in a nice smooth manner) when i aplied power on a corner........my gf loved the car so we bought it. as we were pulling away the diff made a bit of a clunk, my girlfriend was horrified that her new car was making such horrible noises but i said it was fine it only does it now and again. by the time we got home (about 100 miles away) it was clunking badly and with each clunk you would feel the plates slip and the steering go light.....it was doing it on every corner now and my girlfriend was nearly crying. The next morning i got some royal purple box oil for it, changed the oil, took the car for a drive and within 5 miles it was perfect. girlfriend over the moon, lsd still working great now 2.5years later (changed the royal purple once in this time) I have a feeling the garage that imported it ghanged the box oil as part of a 'service' when it came into the country and just used cheap oil.
Experience 3, similar to experience 1, this time i bought a recon ep3-r box with new spoon lsd but was already filled with MTF. knowing how Honda syncro's like the right oil, i decided to try it with the mtf....was same as my previous bad experiences and i honestly didn't think i would manage to bed in the LSD doing figure 8's without snapping a shaft or popping a cv joint. after a lot of research, i thought i'd get the royal purple (because i can get it same day off the shelf) just to bed the diff in while i do some more research on something that works with the syncro's, aswell as the LSD, and get something ordered off the internet.......thats the stage i'm at now.
I imagine on a 'race' car the syncro's wear out pretty quick and you need to preserve them as best you can( i imagine getting a bad grinding box will affect your lap times) where the lsd will be expected to be rebuilt after so many races anyway and wont suddenly wear out during a race.....on a road car the lifespan of the lsd is more important though, for me anyway, i don't enjoy pulling gearboxes out any more often than i need to.
I'm deffinately going to try an mtf with friction modifier on my next oil change, but if you are using your car on the road and need something in your box quick then i would deffinately prefer a synthetic 'gear oil' that lets my lsd work correctly and nurse my syncro's until it's upto running temperature (i mean you shouldn't be ragging your car when cold anyway. The box oil heats up and thins out before the temp gauge starts moving) than use any kind of normal mtf if you don't have the friction modifier.
will a mtf + modifier give the same long life of your LSD (road use) as a synthetic gear oil? i hope so.
also,i don't really think mixing mtf and gear oil is a good idea......say for example a synthetic gear oil is 5% friction modifier and 95% gear oil, and 5% is the ideal amount of friction modifier for your lsd (just a figure i have plucked from nowhere)..... Then if you do a 50:50 mix then all your'e doing is making up an oil which contains 50% mtf, 47.5% gear oil and 2.5% friction modifier when the ideal for your tranny is 95%mtf, 5% friction modifier.........the 47.5% gear oil in your mixture would be 'diluting' the effects of the mtf and friction modifier.
this is all assuming mtf + modifier works, i've read it does but never tried it personally+