Differential Oil, Synthetic vs Conventional
 

I have a 1998 2500 Dodge, 4x4. I pull a boat and a toy hauler. I only put about 4000 miles a year on it. I am assuming the differentials came with conventional oil. Would I be better off changing to a synthetic? Would I need to flush the axle housings?

Synthetic runs a little cooler and is supposed to last longer.

I ran mine up to operating temps, drained off, wiped out the bottom and replaced with Royal Purple 80/140 IIRC.

Welcome to the site.

Hello! My experience with synthetic oil in the LSD differential of the Beast was not a happy thing. Do they put LSD differentials in a 4X4? It was just too slippery and caused a lot of slippage and "chuddering", especially on LH corners and turns. I had opted for the synthetic oil in the diff as we tow kinda heavy with the RV that we drag all over N. America. Anyway, went back to dinosaur oil and have not had any slippage of any kind since. As always, YMMV.

Cheers!

Mike

The shuddering is actually lack of slippage, although some synthetic oil manufactures say that friction modifier is not required if it's nibbling a little adding 4 oz of friction modifier to the differential would have probably taken care of it.

Thanks guys for your input. Some other sites recommend the Royal Purple with friction modifier. Might be better off to stick to the original conventional with modifier.
Thanks again!

I should have stated this in my original post. I had put in 4 oz of OEM friction modifier when I changed to synthetic. When I noticed the chattering et al from the diff my wrenching pal said to add another 4 oz of friction modifier so I tried that. It still chattered like crazy and that's when I went back to regular dinosaur diff oil with one bottle of modifier. No problem since. This is my experience, yours may be different.

Cheers!

Mike

That's too bad you couldn't have kept the synthetic in the diffs especially with our cold weather, it does make such a difference. I use Amsoil and never add any friction modifier.

I have been running Amsoil 75/140 without and modifier in my Dana 80 for 300k. Seems to do just fine, everything inside looks good and OEM bearings feel great.

The Royal Purple I used contained FM so I didn't add any. My rear didn't chatter and will still lock up occasionally after spinning one tire a bit...

Pretty much been in there for almost 80 K miles...

"Generally" synthetics will run a little cooler than regular gear oil. That doesnt mean ALWAYS either but generally. And that said, 140 will run hotter than 90 because of the energy needed to move it and the lack of dissipation compared to a thinner fluid. Within the past year I've had numerous conversations about gear oils with oil company techs when I was comparing the temps of my differential after installing a temp gauge on it. Interesting stuff to say the least. One tech from a specific manufacture was even trying to get me to run 80-90 instead of 75-140 claiming it would reduce the overall temps even understanding that I tow a 14k-15k 5th wheel in the summer and in an area that can have some substantial hills. These guys have lots of cool info which the everyday consumer doesn't care about or need to worry about. End result, I currently run Royal Purple 75-140. If I lived where the ambient temps were much cooler then I'd probably run 75-90 or 80-90. If I lived where 0* in not uncommon then definitely the lower viscosity.

And in regards to LS carriers, most synthetics have friction modifier included in the product so no need to add more unless chattering is a problem.

I switched over to the Amsoil about 2 yrs after I bought it. The oil application guide recommended a can of the friction modifier (4oz i think) and I put that in. That was 12 yrs ago and I'm still running the same oil and still no chatter even on tight turns other than wheel chirp when I'm empty and on the pedal.
I guess I should change it again but it is a round two job at this point and I haven't gotten around to it.