Yes or no? One gear manufacturer says to not use it. Use conventional. Just filled mine with Ford synthetic oil and Ford friction modifier. Haven't driven it yet. Its GL5. I also see mixed recomendations on using synthetic in toploaders. Went with conventional in it. Also mixed recomendations on the top loader. Not sure why synthetic would not be fine in the 9" Traction lock. Saw something about heat destroying it. I thought synthetic was supposed to hold up better under heat. Why am I confused?

Yukon made the gear set for the rear end. Found the paperwork from them and they say do not use synthetic. oil. Drained it and put in regular G5 85-90 with aditive for limited slip rear ends. Any answers from anyone as to why synthetic will damage the 9"?. When I changed the rear end oil in my Dodge Dakota last year Dodge recomended synthetic oil with aditive. Anyone?

From what i understand in this synthetic vs non synthetic fluids. Synthetic fluids will not dissipate the heat from the fluid as good as non synthetic fluids. 
Possibly causing a higher temperature on the system. Just my two cents worth. Joe 

Located in Harford County Maryland 

Isn't Gl5 fluid synthetic??

John - "The Hoosier Hurricane"

I think GL4 was the last non synthetic. All GL5 I have seen are synthetic. The rear end shop that built my third member said synthetic was fine as long as the axle seals were new. It's an open non locker.

GL-5 is available as synthetic or non synthetic.  Maxima is one manufacturer that produces non synthetic GL-5.  I think Motul does also.

With my Detroit TrueTrac, non synthetic and without friction modifiers is recommended.  Without looking back at the owners manual, one thing those would do is reduce the bias ratio.  This is a helical gear differential and not a clutch differential.  On a clutch differential, synthetic could have a similar effect.  Refer to the owner's manual.


Lawrenceville, GA

Many years ago I spoke to the technical department of an oil manufacturer. The discussion was on transmission oils. It was explained to me that the advantage of a good synthetic in those situations is if there is a problem with extreme high temperatures. The example was given to me of some heavy truck transmissions where mineral oils were not surviving due to the extreme temperatures being generated,the oil would break down and transmissions were failing. Changing the oil in these vehicles to synthetic fixed the problem.
 The answer to my question at the time about running the expensive synthetic was, unless there was a problem with extreme heat being generated for some reason then I would be wasting my money. This was coming directly from the person working in the technical department of the oil manufacturer. 
 From my experience, synthetic does not always mean better, with engine oils at least. For example, I bought some synthetic on one of my trips to the USA and I put in the trunk of the car I was sending back to Australia. The synthetic worked out much cheaper than the quality mineral oil I was using here. It was one of the big brands, Castrol, Mobile, or Valvoline, I can't recall what it was now, and it was junk. It burnt much faster and the oil level dropped quicker than it ever did with the good mineral oil I was using. 
There is quality synthetic and there is junk synthetic. I suspect it has lots to do with marketing. If it says synthetic people assume it is better. 

Early on I did a lot of Ford and GM rear gear work.  My experience with synthetic oil in the rear differentials was a propensity for oil leaking from the front seals when using synthetic oil.  Comebacks were common early on where synthetic oil was being used as the front seal was usually changed out again thinking it’s just a defective seal.  With the replaced seals leaking a second time, then switching back to a conventional oil always stopped those front seal leaks.  It didn’t take long to figure out that those leaking front seal problems were the result of using synthetic rear gear oil.  There were enough instances with those oil leaks that I would simply recommend to customers to use a good grade of conventional gear oil.  Problem solved.

Lorena, Texas (South of Waco)

Any theory on why it would leak with synthetic?

From what I was told some synthetics have additives that softens the rubber, others have additives that harden it..