Can you have too much friction modifier?

[AndrewIdaho]

Another newbie question: I am about to start the work on the gas tank and fuel system and figured it was an opportune time to fill up the IRS differential I received from FFR while it is easily accessible. My reading of various threads is that the FFR differential is prefilled and that a top up with gear oil would all that would be required.

Well I topped it up with the Motorcraft SAE 75W-85 Synthetic Hypoid Gear Lubricant and it took about 26 oz or about what I calculate to be 50% of the total capacity of the differential. I did not smell anything that seemed offensive so I question whether it has enough friction modifier.

So as to not add too much friction modifier, my inclination is to purchase a 4oz bottle of the Motorcraft Additive Friction Modifier XL-3 and add two oz (after draining a few oz) and then topping up with the gear lubricant.

Note that after about 500-1000 miles of break in time, I was planning to drain the differential and fill it with new gear lubricant and friction modifier in the required quantities.

Any concerns with this approach?

My thanks in advance for you comments and advice, Andrew

[rich grsc]

There is ZERO need to drain "break-in oil". Have you ever had to take a new car in for a break-in oil change? There is nothing to break-in
If you'l sleep better add the friction modifier, doubt it's needed, and won't hurt anything.

[CraigS]

I don't use any modifier. I use Mobil1 75-90LS. The "LS" indicates it is ready to be used in a limited slip diff.

[Dgc333]

I wouldn't add any until you drive it. If the rear end chatters and is not smooth going around corners then add some.

Too much friction modifier can cause the differential to slip going straight negating the whole reason for having a limited slip differential.