Friction modifier or no...?

[RF Overlord]

My '03 Mercury Marauder w/8.8" LS calls for conventional 80W-90. When I switched out the stock 3.55 gears for 4.10s last year, I put in Mobil 1 75W-90. So far I've been very pleased with everything, but I am now in the process of installing a supercharger and it has been recommended to me to use 75W-140. I have some Motorcraft 75W-140; my question is whether I need to add the Ford friction modifier to it? Also, Ford has 2 or 3 different part numbers for FM and I was told (some time ago) that one was for use with their conventional, one with their full synth (like the 75W-140?) and another for use with their "fuel efficient" gear oil..."I'm so conFUSED"

...can anyone shed some light on this?

 

[Big Jim]

If you have a factory LSD then you need to add friction modifier.

Ford has two part numbers for friction modifier. XL-3 and XL-7. XL-7 is the one specified for synthetic uses. I have used a lot of XL-3 in synthetic applications before SK-7 was introduced with no ill effects but there must be a reason for adding the product.

 

[JHZR2]

Id prefer a gear oil that has the LSD built-in at the blender.

Redline has the LS additive in the lube, and sells the same stuff, so mixing is totally fine.

Motorcraft will likely be the same thing, but I think redline will be a better gear lube.

Go with redline, and add more redline bramd LSD additive if your lockup isnt right.

JMH