Changing to synthetic lube for the rear?...

[Rory428]

On both my drag cars, both 4 speeds, I have been using Redline "Heavy Shockproof" synthetic gear oil in the rearends. Both cars have spools in them. The Redline oil is much thinner than conventional 80/90, but it has been proven to be up to the task of years of 6000 + RPM clutch drops. I also use Redline Shockproof oil in both my Jerico 4 speeds.One interesting trait of the Redline gear oil is the fact that it clings very well, instead of running off over time. A few years ago, I pulled the diff cover off my Mustangs 8.8 after the car sat all winter without moving. Despite several months of sitting, when I removed the cover, the entire inside of the diff casting was still fully coated with the lube, which is a great thing for vehicles that sit a lot, protecting the gears and bearings from getting surface rust. One thing to be aware of, is if you spill some, it is a real pain to clean up.  About 6 years ago, I spilled a bit on our concrete patio, and the stain is still there! Even multiple sessions with  a pressure washer failed to remove it.

[TorinoBP88]

That might oil sounds scary.

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

[TorinoBP88]

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[Falcon67]

In a road car I would use 90w-140.  In the street strip cars here I use 75w-90.  Don't have any problems.  Cars sit a lot, never had rust issues or abnormal wear problems.  I use Valvoline synthetic, I don't really believe in paying a hefty premium for some of the "specialty" brands. 

[machoneman]

I run 75-90W dino in my 9" rearend yet unlike a locker, it has clutches. I'll venture that most/all gear driven center sections can run synthetic oil but any that use clutches must use old school dino oil with a friction modifier.

[babybolt]

I saw an engineering report once about synthetic gear lube.  The report was mostly oriented towards operational HP loss or efficiency.  The synthetic did help some.  I asked the engineer what he thought about the synthetic, and he thought it was OK, but he noted that the synthetic didn't transfer heat away from the gears as fast as dino oil.  But then this was back in the 90's and there have been newer blends of the synthetic since then.  He also did some tests on fill level, and there were some significant gains by lowering the oil level in the differential 

[cjshaker]

I run 75-90W dino in my 9" rearend yet unlike a locker, it has clutches. I'll venture that most/all gear driven center sections can run synthetic oil but any that use clutches must use old school dino oil with a friction modifier.

Bob, the 75w-90 Valvoline Synthetic that I have is actually rated for Limited Slip differentials.

[machoneman]

Thanks Doug, good to know. Now that I think about it, last time I fooled with my rearend I did see various syn. based gear lubes that had some kind of friction modifier added. But, I shied away as they seemed to be all GM oriented and didn't mention Ford's Trac-Loc. I instead went the dino route and added that little bottle of Ford specific friction modifer. All's been great since.

Btw, I do hesitate to throw syn. oil, not at the engines, but stick trannys (and rear ends). I had recounted this tale here long ago and on the old FE Forum as Rory can also attest!

My new Richmond 5-speed in the '70 Mach 1 started acting up with few miles on it. Even talking to a senior tech fellow with Richmond Gear, and no kid himself, I was assured the syn. oil based light auto tranny fluid was as fine! Turned out he was dead wrong!

The hard shifting went away after a drain and refill with good old 75-90W dino oil! Any old school design tranny with brass blocker rings MUST use dino oil as syn. oil is too slick to allow the slider to engage those rings on a shift. This makes sense when one realizes that carbon fiber rings (or, put another way, non-brass rings) in modern trannys can and should use syn oil. I learned the hard way that even the tranny maker can be wrong at times!


     

[Nightmist66]

Thanks Doug, good to know. Now that I think about it, last time I fooled with my rearend I did see various syn. based gear lubes that had some kind of friction modifier added. But, I shied away as they seemed to be all GM oriented and idnt; mention Ford's Trac-Loc.

My new Richmond 5-speed in the '70 Mach 1 started acting up with few miles on it. Even talking to a senior tech fellow with Richmond Gear, and no kid himself, I was assured the syn. oil based light auto tranny fluid was as fine! Turned out he was dead wrong!

The hard shifting went away after a drain and refill with good old 75-90W dino oil! Any old school design tranny with brass blocker rings MUST use dino oil as syn. oil is too slick to allow the slider to engage those rings on a shift. This makes sense when one realizes that carbon fiber rings (or, put another way, non-brass rings) in modern trannys can and should use syn oil. I learned the hard way that even the tranny maker can be wrong at times!

Any limited slip diff. from the factory for the last couple decades at least seem to use synthetic. The Motorcraft friction modifier seems to be the best that we have found. I can't see getting the extra parasitic drag from 75W140, when 75W90 seems to work just fine with the Trac Loc and a stick. JMO


I agree with your point on the synthetic and regular brass blocker rings, that's why I only use plain old 80W90 Mystic in my toploader.

   

[Qikbbstang]

FYI Ford Gen I 93-95 SVT Lightnings came with Synthetic in their factory 4.10:1 rear axles. Said trucks also came with a deeply finned/reinforced 8.8 axle housings - 100% unique to Lightnings/nothing else got that housing. They also sell/sold a special syn. friction modifier for the locker that resides within.