I tried this in the 87 to 96 F series forum with not much response..... Maybe you guys have a clue.....
I was talking with a Ford truck enthusiast the other day and he warned me of a potentially rear end killing problem with different differentials.

He said that there are two rear ends, one takes regular gear oil and the other takes synthetic. And if you put synthetic in the regular rear end you'll kill it within a few miles. And I need to find out from ford which differential I have and use the correct fluid.

Is this the limited slip additive or something else?

Does anyone know what he's talking about?

 

I believe there might have been a misunderstanding, or the individual that you recieved your information from is mistaken.
Mineral gear oils and synthetics are both a hypoid gear type lubricant and can be used in any differential. Synthetics are used and recommended for their performance advantages over mineral gear oils. These synthetic gear oils tend to gum up less and oxidize at a much slower rate. These also provide use in a wider range of temps, and can maintain a more uniform viscosity in these varying temps.

Ok, if you want to run synthetic in your rig, run it, if you dont, dont. It wil make little difference and your gears and bearings will survive.

Friction modifiers are completely a different subject. This additive is a conditioner for the clutch plates used in limited slip differentials. Failure to use these conditioners will cause the clutches to chatter during turns. Thats about all that will happen to them, but the friction modifier is a seperate additive.
Some off the shelf gear oils are said to contain the modifer, but experience has taught me that they are not adequate, so I always use factory bottles of friction modifier.

 

That is BS. While newer rear ends usually use synthetic, they will work with conventional oil, and vice versa. As a matter of fact, the 95 factory manual mentions the option of using synthetic instead of traditional gear oil.

 

The fellow didn't have specific details, only that the wrong fluid destroyed his rear end.
But from the sound of the limited slip additive needs, I'm guessing his was limited differential and was missing the proper additive. I think he may just be a little fuzzy on that detail. He didn't remember the difference only that it was significant.

Thanks for sharing knowledge....

 

In some cases, and only in some, there can be a compatability problem when synthetics are used and then the switch back to mineral oils is made. Startting with mineral oil and switching to synthetic is almost never a problem, but there could be some issue when the synthetis is used and a non capatable mineral oil is introduced.
Sometimes the two oils will seperate from one another leaving a friction surface exposed, and will undoubtedly lock up, but this is an extreme case and quite often considered very rare. This is probably the exception and not the rule, so read about compatability and you should be safe.
Axle bearings are rather durable, large and strong. These robust bearings can an will function with very little oil so finding one that burns up in a few miles after any oil is introduced is not common at all, and might suggest a problem with the set-up prior to the axle failure.
The theory that the failure in the above decription was a result of the new fluid that was introduced is probably just a coincidence and the failure was going to happen regardless of the fluid that was being used.

Failure to use friction modifier will alos not bring immediate failure to the axle, as this would only cause the clutches to chatter, and this chatter probably would not create much heat, or at least enough heat to cause a failure. I have run several LS diffs with no friction modifier. The purpose is to keep as much tension on the clutches as possible without permitting them to slip. Different applications for sure, but this does not cause a failure such as the one described.