Changing to synthetic lube for the rear?...

[cjshaker]

I am considering changing to synthetic gear oil for my Detroit Locker rear. Are there any drawbacks to using synthetic with a Locker? Also, I was wondering what you guys would recommend for viscosity? I have a 5 gallon bucket of 75w-90 and was wondering if that would work ok. I probably wouldn't have any qualms about using it for strictly street use, but I'll be doing a fair amount of racing this year and not sure if it can handle the shock loads.

And while I'm on the subject, is anyone running synthetic oil in their roller cammed engine? I know it's not a good idea with flat tappet cams, but what about a roller cam? Besides increased chances of leaks, I don't see any drawbacks once the rings are broken in. Does an engines bore finish need to be adjusted for synthetics?

[My427stang]

No worries on either in my experience.  Even the leak discussion is really only due to the detergents in early synthetics and sludgy rope seal motors, not sure if that is a real threat anymore

I am not sure there is any benefit though.  However, I do buy synthetic blend gear oil because the difference is so slight in cost that somehow I feel I am doing something better

I have never seen or heard anyone changing cylinder prep for synthetics and once the rings are seated in a running engine I cannot imagine it would be a big difference anyway.  I'll even go that far for flat tappets assuming proper break in is complete.  I have had lots of semi-synthetic through flat tappet motors with no issues.  I say go for it with your roller motor if you want to try it

LOL so which side of the fence am I standing on?   both I guess and hopping back and forth

[Nightmist66]

I am currently running Schaeffer syn. 75W90 and some Motorcraft friction modifier in my car with a Yukon Trac Lok. We have also run syn. with a locker and not had a problem.

[jayb]

I'm interested in this question also, with respect to the rear end lube.  I got some Richmond synthetic lube free as a promotion with the last set of gears I bought from Summit.  I drained some out of the bottle to look at it and it looked the same as ATF, which didn't fill me with confidence.  So I reverted back to my standard gear lube for last year.  I have more new gears to try this year, so I'm thinking about trying it this year.  I'd be curious to hear from anyone with experience with that stuff.

[My427stang]

All the modern big diesel pickups require synthetic in the rear, not sure why they'd add the extra cost if it wasn't required for heavy work.  However, that is also very thick, nothing like you describe.  I'd be afraid of something thin working it's way out of the axle seals with deflection out at the end of the axles. 

That being said , if Richmond is providing it with their gears, hard to say it won't work

[plovett]

I've been running synthetic oil with my flat tappet cam for years.

As for the rear end, I don't know the answers.  I'm guessing it'll work fine.  I'm subscribed anyway. 

paulie

[FirstEliminator]

We do a bit of rear axle work at my shop. I can't think of any rear in a vehicle that is built within the past 15 years that doesn't recommend either 75/90 or 75/140 synthetic in the rear. Front axles still call for 80/90 mineral oil. When I worked at a Ford dealer back in 03-05, from the factory the rears would be filled with 75/90. To help aid in noise reduction Ford had a TSB that suggested going to 75/140. I believe there are a few reasons for the synthetic in the rear. One would be it may have a slight reduction in drag in comparison to mineral oil to help efficiency. The other is it's probably a bit more durable to stand up to heat from heavy loads. Also, it's supposed to last the life of the vehicle, reducing the waste from an oil change.
    One thing you can try for comparison is to make a 1/4 mile run with mineral oil and immediately measure the temperature of the rear. Then switch to synthetic and take temp readings. From what I hear, the syn will run cooler. 

[RJP]

All the modern big diesel pickups require synthetic in the rear, not sure why they'd add the extra cost if it wasn't required for heavy work.  However, that is also very thick, nothing like you describe.  I'd be afraid of something thin working it's way out of the axle seals with deflection out at the end of the axles. 

That being said , if Richmond is providing it with their gears, hard to say it won't work

Don't know about pickup trucks but Class 7/8 big rigs use and specify synthetics in the drive lines to qualify for a 1 million mile warranty. Use of petro based oil the warranty is usually 500K. Gear Mfgs. have a lot of faith in synthetics. My experience with synthetics is somewhat limited but it made a big difference in the Casale V-drives in 2 of my boats. Allowed a couple hundred more rpm than the 85-140 I ran before switching to Red Line synthetic 75-90. I've run the same Redline 75-90 in a couple of my cars with no noticeable difference.

[rcodecj]

Synthetic is all I've ran in my 9"rear ends for decades.
My rear ends are traction locks though.
My newer mustangs with 8.8 rears used synthetic, I think my 2000 F150 uses it too from the factory.

I run synthetic engine oil in my flat tappet and roller engines, again for decades.
What I wonder is, is it ok to run a higher zinc synthetic in a roller cam engine, say around 1000 ppm.

[Drew Pojedinec]

My tru trac rear end specifically calls for mineral gear oil not synthetic.  Otherwise I would totally use synthetic.

[GJCAT427]

I use synthetic blend in my 95 F350 powerstroke sterling rear and I like it in the winter. It doesn't seem as stiff at 10 degrees. I first put it in when the rear wheel seals started leaking at 80,000 miles. The truck now has 182,000 on it and I need to change the rear cover due to rust, but its still quiet and smooth.

[cjshaker]

Well that makes me pretty confidant that it won't be an issue in the rear. I don't see how it could possibly affect the locker, and apparently they have a lot of confidence in it handling high loads, although the shock of a 4 speed and slicks would be the ultimate test. I guess we'll see.

I drained some out of the bottle to look at it and it looked the same as ATF, which didn't fill me with confidence. 

Jay, I hadn't opened up the synthetic that I had, so your comment left me curious. I poured out a small bit and felt it with my fingers and it seemed about the same consistency of a 30w oil. Certainly not as viscous as regular ATF. Although it is rated for use in newer transmissions, I don't think it works very well with old brass blocker rings. I wonder why your fluid feels so different? Mine is Valvoline Full Synthetic.

[rcodecj]

It's been a while but my synthetic rear gear oil was more like 30w engine oil too.

[jayb]

I guess that is what had me concerned, the viscosity was so much less than regular rear end lube.  It did seem to be the same color and viscosity as ATF to me.  I will probably try it this time around and see what happens, just seems odd to be putting such a lightweight oil into the rear end...

[ScotiaFE]

Here is another oil test. This one is on rear gear oil.

http://www.lastgreatroadtrip.com/wp-content/uploads/2010/11/gear-oil-comparison.pdf