The Basics:

Friction Modifier (FM) is slippery stuff. The FM is in place to reduce LSD clutch chatter upon engagement. The amount needed is relative to the application and the age of the differential. In full race cars, you want the clutch to lock up nice and strong while not being so concerned with a little chatter. In high mileage (or worn) LSD cars, the clutches will be worn and need less FM to attain the desired effect. The downside, or catch-22, is that the syncros don't need or even like the FM. In most cars this is not a problem because the differential and transmission are separated and do not share the same lubricant. Our transaxled P-cars do share, so we must prepare for that.

FM options:

FM needs to be used in LSD cars, we know that. It's the amount that is variable. As a general rule, what comes out of the bottle is sufficient for most applications. Some folks will mix FM stuff with non-FM stuff (75W90NS and 75W90) until the desired effect of "no chatter" is attained... the reason for this being again, that syncros don't like FM.

Oil weight:

In high-performance applications where the temperatures may get higher than street use, it is common practice to use a bit higher weight oil (75W90 vs. 70W80), and higher still if the tranny/diff is not externally cooled (80W140).

What about GL-5 vs. GL-4:

The main complaint against GL-5 use in syncro'd trannies is because the chemicals used to provide the extreme pressure protection can be corrosive to synchronizers, which are commonly made of brass or bronze. Here's what the 944 factory manual has this to say: "Filling capacity Approx. 2.0 liters hypoid transmission oil SAE 75W90 to API Class GL5 or MIL-L-2105B, or SAE 80 to API Class GL4 or MIL -L-2105."

Which one is right?:

In the end, it is up to the owner. Though statements of GL-5 use in a syncro'd tranny are somewhat against... history and sales of GL-5 lubricants (like Redline 75W90) continue to be favorable. This is a heavily debated topic, even within the industry. The only things we can assume for sure is: 1. Don't use FM in non-LSD transaxles 2. Don't add FM to make the clutches lock up sooner in LSD equipped cars.