The less friction mod you add the more "bite" the clutches will have. The more you put in the more slippery the fluid gets and the less the clutches "bite". As others have stated, some fluid already has additives, some don't. If you have a fluid that doesn't have the friction mod built in, go to your ford dealer and buy a 4oz bottle, should be a just a couple bucks and that's all you need for one fill. Big thing you wanna make sure you do is use 100% synthetic oil - and these cars call for 75w140 not 90, there was a tsb (either for us or some other 8.8 car) a couple years ago and Ford switched to the w140 stuff in the stangs, at least.