well i had my new eaton posi installed in 2004 and the manufacter said it needed mineral based oil,, my mechanic said just run synthetic with posi fluid in there so i did,, every year i drop the oil and theres some metal on my plug magnet which made me decide this year to run regular rear fluid with posi additive,, now since ive been runnin sythnetic for 4 years is this going to cause a mixing problem with the new non syntetic oil?

 

NO! Small amounts of metal (shavings) are completely normal.

 

what about mixing the synthetic and non synthetic fluids is that a problem?

 

No. Synthetic oil is still OIL!! Just "purer", i.e. more precisely manufactured.

 

I though sythetic was "man-made" stock. but def can be wrong
if they state only mineral oil i would do exactly what they spec. There is a reason they state no sythetic.

 

I dont know much about the Eaton, but Ill share what happened to me:

I had an AWD Astro van, with a clutched front posi differential, similar to an auburn. I dont know what they are called from the factory. Anyway, the seal went out, so I decided to change it myself. All went well, untill I went to put fluid in. I asked the guy at the counter of Auto-Zone what the factory recommends. He said 90 weight gear oil, but that since I was changing it, use synthetic. So I did.

To make a long story short, the synthetic fluid actually "glued" the clutches together. I took it to the dealership, the only people who would work on it, and it had locked the front differential, acting like a spool. It had busted the mounts, and the housing, and cost me over $4k to fix, with used parts. If it has any clutches, DO NOT use synthetic fluid. Thats from a GM certified master mechanic, so take it for what its worth. And I no longer deal with Auto Zone.

 

thanks for the input,, but theres no way synthetic oil glued gears,, dude you musta had a screwed up bearing or something,, but besides the point i been runnin synthetic oil in the rear for 4 YEARS now and no problems,, im just gonna leave it be now,, IF IT AINT BROKE DONT FIX IT YA KNOW

 

"man-made" and "manufactured" mean the same thing. It's still has the exact same hydrocarbon compones as petroleum oil.

 

It will make absolutely no difference to your Eaton Posi whether you use syn. or petroleum based oil as long as you use the GM posi additive. Go look at the Eaton web site! The reason Auburn recommends using petroleum oil is because the unit was never been "certified" for use with syn.

 

thanks z28sorr im just gonna continue to run the syn in that case