What is the purpose in using synthetic diff fluid, why does the manufacture require it?? My new chrome diff cover is leaking so im getting a gasket for it this week and im tired of paying for synthetic diff fluid. I change it once a year. And this summer im installing a posi. Is it ok to run valvoline 85w140 regular from advance?

 

Why on earth have you been changing it once a year?? That's the first foul. Second foul is the chrome cover. Chrome dissipates heat horribly, and it's out of style, LOL. Why. It just catch it when you take the cover off and reuse it? Done that several times over the years.

 

Darn auto correct on the phone...meant to say why not just catch it when...

 

I tow a lot and offroad, puttin a boat in the water the rear diff goes under water. Your suppose to change it when that happens. And I just like to keep clean fluids in my truck. I like the chrome cover its a lot thicker more durable cover than the stock one.

 

You can use the Valvoline if you like. Just make sure it is a GF-5 so you get the max in additives. FWIW, if you have a NAPA store around I'd price compare as the NAPA is made by Valvoline and is the same product. FWIW #2, Valvoline is no longer the maker of NAPA motor oils- Mobil is.

In regards to changing so often. Steven's got it right. If you tow a boat and the axle gets covered with water, you should be changing once a year regardless of mileage. Reason- the axle will be hot when you launch from the distance driven. When immersed in water, as the axle cools it develops a vacuum in the axle and will suck water past the seals. It's not much but you also have the alcohol based friction modifier in the mix which will also wick water. Eventually you'll overwhelm the fluid with water and corrosion will set in- not a good thing. You should be changing any axle that uses a friction modifier (T-Loks) at least every 30,000 miles or 3 years. The modifier will wick moisture out of the air over time and cause corrosion.

 

So your sayin as long as its a GF-5 u don't "have" to use a synthetic diff fluid ?

 

Absolutely...would have been helpful to have that info in the original post.

 

Now that one's new; never heard that one before!

 

I didn't know that either. My new 2000 F150 T-Lok clutches started chattering around 80k miles (about 4 yrs) and I replaced the fluid. Had to use 2 bottles of modifier to get the clutches to quieten. So when I bought my 2002 used 3 yrs ago (with 69k miles), the clutches were quiet and my philosophy has been to leave it alone until the clutches start chattering. It now has 113k miles and the clutches are still quiet.

Keep the tips coming! Thanks!

 

If you do switch to an aftermarket diff, depending on the manufacturer some require mineral based GL5. It will come with a huge note in the box if thats the case. The most common mineral based weight is 80w90. I think almost all of the late model ford stock diff's call for 75w140 synthetic. BTW synthetic GL5 eats RTV sealant.

 

I did a lot of homework when I thought my rear end noise was the diff going bad. The only diff I ran across that recommends against synthetic gear oil was the Eaton Tru-Trac LS. There may be others, but I didn't find any. Any LS with a clutch and I'm thinking any Posi (wouldn't swear on that one but seems logical) should be a-OK with synthetic.

 

An Auburn unit will chatter like crazy with synthetic. They rec only mineral based with there units.