Synthetic Differential fluid need Friction Modifier?

Not true. Assuming you have the stock rear end on your GT your T-Loc limited slip differential requires the friction modifier for its clutch disks.

Actually, a lot of the synthetic gear lubes profess to have friction modifier built into the gear lube. It goes brand by brand and weight by weight (in the synthetic brand I run, 75-90 has FM but 85-140 does not).

Even if you contact the fluid manufacturer, it's no biggie if you are not happy without the modifier - your rear would bite too hard (not enough differentiating) so you could add FM down the road. It's only about 4 oz of juice.

Good luck.

Put it in. You will only be saving about $4. I don't care what they (Syn Cos) say I put it in.