help with Gear oil for 3 speed and 9" for fuel mileage

[gregamorton]

Guys I have been reading up on gear oils and am getting lost in all the jargon! My truck is tore apart and going back together little weekly. 300 3 speed 3.03 tranny and 9" with hopefully 2.75 gears. Im shooting for fuel mileage and least amount of drag for a true high speed cruiser. Truck will be used for cruising and never hauling or pulling trailer. Im looking at all the 75/90 synthetic oils and apparently there is a lot of difference in there thicknesses of each brand. I want to go with thinnest oil that will work great for my setup.

[Ranchero50]

This is a hard one, remember this. Different oils perform different tasks differently.

What you need for a manual transmission is an oil with excellent shear stability, some anti foaming properties that will climb the gear train in operation. I would not get too hung up on viscosity if you can find an oil that will perform the duty. A straight synthetic 50 weight should have similar performance as the 75/90 dyno in that application.

My '68 fairlane fastback was good for 20+ mpg if I kept it below 60 mph. 289 C4 and 2.73's on 14" radials. I'm not sure if a trucks profile will allow anything close to that though.

[gregamorton]

Gotta keep in mind my 4x4 bump gets 7mpg and my Diesel Mercury gets 13-14 so I would be stupid happy with over 16mpg on Petro! I would love a true 18mpg truck.

[Nitekruizer]

I always ran 80W-90 in my BW T-85N and was able to get respectable gas milage for a 390 FE. My first car, a Chevy, had a 3-spd trans in it and I ran it with 80W-90 as well. However, in cold weather, that trans was almost impossible to shift. So I thought I would try draining the 80W-90 out for winter. I'd fill it every fall, right before winter with SAE 30 engine oil. It made it a lot easier to shift during winter and it didn't seem to hurt the tranny. I always changed back to gear oil in the spring.

I would say though, that whatever you try you're kind of on your own, in uncharted territory, in other words. Some manual transmissions even call for ATF instead of Gear Oil. There's really no way of knowing for sure if you'll damage your trans. Probably not, but who knows. I took a chance by doing what I did and you basically would be too.

[gregamorton]

I myself was wondering about 75% aft and 25% Lucas for auto as the binder to coat the gears?? I wonder if this would be thinner than the 75/90 synthetics... I don't under stand the papers over the gear oil Ive read. One thing I have figured out from the papers is that the Lucas 75/90 synthetic I used in my one ton 4x4 bump apparently did not pass half the tests in the studies!! So much for both Dana 60s and the np205 transfer case that just got all fresh Lucas!!

[Ranchero50]

We run Mobil stuff at work. I honestly don't understand how the smaller 'specialty' companies can have a R+D and Q+C group nearly as capable as the big guys. I'd lay money a solid 99% of their product sales are hype and not actual product quality based. The Mobil engineers and sales guys we deal with are surprisingly honest about this stuff. They have the data (both good and bad) showing what each of Mobil's products do well and not so well. We send them oil for analysis and they tell us how what's going on in the gearbox and what we should do to the oil to improve or maintain it's performance.

Along that thought on running ATF and Gear oil together, just be aware of what the ATF does well and what the oil does well. Honestly I think you should run what's suggested and change the viscosity vs. go to another style product. Most hydraulic oils don't lube well and the gearbox is really going to want more shear protection vs. a viscous transmission.

Just some thoughts.

[gregamorton]

Well do they run any thing thinner than the 75/90 gear oils? Just curious as ive not seen any thing else. My dad used 90/145 i do think in every thing!

[Ranchero50]

Understand that oils are graded on viscosity or pour rate of the fluid. The split number oil's flow differently cold or hot based on the additives. The irony is the synthetic stuff is supposed to flow better but is still graded like regular oil.

If you could find a 10wt hydraulic oil with the correct shear and lube properties, you could run it in your transmission. The heavier oil acts like a cushion between the gears and will run quieter. As a side function it also takes more energy to squeeze out from between the gears. Modern tranny's are built tighter to run thinner oils without the noise (well, except for the ZF 5 that sound like crap running ATF).

Basically what I suggest is drop some 50wt in it and see if it's too noisy for you. If so go up to 90wt.

[gregamorton]

50wt?? Brand and recommended what to look for as I've never seen any that weight.

[tnlprt]

On the right grade of transmission lube


http://www.davidkeetoploaders.com/specifications.htm

[390bump]

I looked into this a while back as I was changing the oil in my RAT 3 spd. The 75/90 gear oil is very close to 40 weight motor oil. So I put in half Mobil 1 gear and half 0W-40 Redline motor oil. Mainly because that's what I had on hand and didn't feel like driving to the auto store... but read up on it and it turns out motor oil is good for tranny. Redline is top grade oil so I wasn't worried about using it. Honda manuals use 30W motor oil and we all know how Hondas last a long time... Anyway tranny runs great. Never been overhauled so it has a slight leak from seals... otherwise its perfect.