Synthetic or conventional for differentials

I’m just curious if most are using synthetic or conventional gear fluid in their differentials and transfer cases. I have a 2009 4WD and I’m planning on using the 75-90w but I’m having a tough time making up my mind if I should use synthetic or stick with the conventional?

 

You should do whatever you *believe* because 50% of the people are gonna tell you conventional is better for diffs and 50% of the people are gonna tell you synthetic is.
Personally, I bet it doesn't matter. What does matter though, is changing it more frequently if you do a lot of water crossings.

 

That’s kind of what I’m thinking. I’m getting older and change is hard, synthetics are everywhere now and I’m just want to make sure I’m doing what’s best for my truck

 

Yeah I hear you on that. I bet it mattered more back when there were clutch plates and such in limited slip differentials that weren't controlled by ABS sensors, etc. What is the primary purpose of the diff oil?

I guess you could argue there's three.

1. Suspend (dilute?) foreign material
2. Cool
3. Lubricate

Maybe you could expand #1 to also include the ability to "happily mix with" water (and still retain #3)?

I don't think conventional or synthetic are going to fail at any of them within the change period, honestly.

 

I had a street legal SCCA ride at one time with a Torsen diff.

75w90 GL5 dino was the spec with 25k OCIs

I changed to OTC syn and kept the same OCI in the years I competed regularly. When I stopped competing, I kept using syn, but doubled my OCI to 50k.

Nice clean diff with no issues at 100k when I sold it.

If you consider your usage to be 'normal', dino is fine.

If you consider your usage to be 'severe duty', you can shorten your OCIs or switch to syn.

 

I don't see 75W90 commonly available in conventional? IMO that's the biggest benefit of synthetic - the technology allows a wider temperature range.

 

https://www.oreillyauto.com/detail/...conventional-gear-oil-75w-90-1-quart/val0/820

 

^Interesting. Never seen it before. No O'Reilly near me.

 

Valvoline isn't O'Reilly specific AFAIK. It just popped on a search.

 

I locally found the conventional 75-90w Valvoline, I wonder if it’s different in different areas

 

^It's good enough.

Valvoline synthetic 75W90 is available in squeeze pouches which are easier to fill with. May be worth a few more dollars for the convenience if you can find it locally:

 

I can't add a whole bunch of "one is better than the other" personal experience. I can speak on how conventional vs synthetic behaved in my Nissan axle, which was a D44 variant made by Dana Spicer for Nissan. The spec fluid was 75W-140 synthetic. I did my first diff fluid change with that. Around 25K miles, I started leaking out of the passenger axle seal. Keep in mind, this is a D44 so it's a simple SET10 tapered roller bearing pressed onto the axle shaft and a seal on the outside of the bearing that wedges into the axle tube. There is a 4 bolt retainer plate outside of those parts that pushes it all into the axle tube and holds it there when the bolts or nuts are tightened down. Quite a bit different than the Toyota designs and far more likely to leak than Toyota.

Anyways, on that axle, I was under warranty so I went to the dealer, which they confirmed was leaking and ordered a new complete axle shaft under warranty which comes with the bearing/seal/retainer. Swapped that shaft after a few weeks of waiting for the part, and was leaking again by the end of the day. I ended up draining that 75W-140 synthetic, and put in 85W-140 conventional/mineral oil by Torco. Pretty thick stuff, but magically never had a leak again in another 10K or so miles before I traded the truck for the Tacoma.

I'm not sure how much it matters in a Toyota diff. I have a habit of buying the same stuff so I stick to 85W-140 conventional by Torco in my Tacoma as well. Runs quiet and the fluid stays looking new for a long time. I've read that conventional can extract heat better from the gears, and that's stated by multiple axle builders so I'm sure there has got to be some merit to it, though I personally have no proof. Maybe it's crap, but I feel better using the conventional for that reason as well.

Some folks argue cold flow of synthetic is better, and that's a fair argument. However - visually, conventional oils tend to cling to metal better than synthetic. The synthetic might drip off like water whereas conventional kind of oozes off. So I'm not sure cold flow really matters, as the parts are likely still soaked in conventional for a long time after operation. So even if the synthetic flows better when cold, maybe it doesn't matter with conventional because the parts stay lubed for longer? I don't really know.

At the end of the day, I don't think either is a bad choice and in an axle like a Toyota that isn't prone to leaks by design, probably either is a perfectly fine choice. I just run conventional because it has fixed problems for me in other applications and it's easy to just keep using what I know. Plus, my gear manufacturer recommends for me to use what I use, so that makes the choice easy for me.

 

^So the factory fill leaked in your Nissan?

 

I bought the truck at 20K, I put in SuperTech 75W-140 synthetic around 21K or so. I’m not 100% when the full leak started, but it was already getting some wetness before I touched it. You can’t really tell it’s leaking until it’s been doing it for at least a little while. The only part I know for sure is that I pulled the brakes after the dealer fixed it and it was literally dripping. Never removed that axle shaft myself again and the drip completely stopped after filling with conventional. Could be luck, or could be the fluid. But yes, the dealer filled it up with their fill with the new axle shaft and that leaked same day.

 

I appreciate all the input. Another problem with getting older and being a DIY person is that it’s harder to make these decisions, way too many options and if you mess something up it’s very costly. I deliberately didn’t bring up brands because I know there’s definitely some different opinions on that.

 

I had Dino in the rear and syn in the rest. Then put syn in the back. Mech lsd.

at the end of the day I don’t really care. They’re regular ass stock diffs. Mostly open.
Back LSD isn’t what you want in a truck. Add to that wear and bigger tires that overpower it

but if I regear and lockers I’d do the same. Or whatever suppliers recommend.

the difference I see is being the guy to change it on time, compared to someone never changing it at all.

 

I use the Valvoline synthetic in the easy squeeze bags....no problems yet.

 

You have a 2009.
It’s use Auto-LSD. There are no clutch packs.
Synthetic is fine. That’s what I’d use.

 

fresh fluid of the right spec (75w90) of whatever brand and syn or not is going to be better than any old fluid. Those squeeze bags are AWESOME.


FYI I went with cheapie

$28 75w-90 Synthetic Gear Lube - 1 Quart (Pack of 2)

75w-90 supertech walmart synthetic last go around and at my typical 30k mile rear diff change... it was still damned good looking to the point im going 40k next go around.

 

Amsoil 75W90 Severe Gear in everything. Not cheap but if you DIY its way cheaper than a dealer change. Did it for cold weather performance and longevity. Did it once around 10,000 miles. Amsoil recommended for normal service change is every 100,000 miles.

Only rear diff had any indication of break in wear with the fluid having a typical metalic sheen to it. Front diff and transfer case were so clean I will probably not need to do them again.