Wow <img src="/forums/images/graemlins/zombie.gif" alt="" /> this is new ...never heard of using friction modifier in the trans before.
May work, I mean it may make for smoother shifting.

Should be no concern for slipping. Synchros in a trans and clutches in a lsd serve totally different purposes.

Give it a try, if it doesn't work, drain it out and replace with fresh gear oil. <img src="/forums/images/graemlins/kewl.gif" alt="" />

'89 Raider V6, AT, SAS'ed, Dana 44 front & rear, 5:38 gears, NP 231 T-case.
'97 Montero SR, cold weather package, 4.90 R&P, BATTLEZ suspension w/ 2" lift, JAOS grill guard & under body protection, custom rock sliders.