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I have my 3 speed overdrive and am preparing it for my truck. When I put it in - what lubricant should I put in it? I believe the book says to use 90 wt. A friend (who has an overdrive in his panel) said he uses Castro 30 wt.

What do you all use 90 wt or 30 wt?

TIA

I may hear some screams for this one, but here goes. We replaced the 90 wt gear lube in my dad's truck 4 speed with a synthetic (Amsoil I believe) oil of the same category. Awesome stuff. The old stuff was super stiff in the winter, while the synthetic was easy to work with. You could push the truck by hand in neutral even at 20 below. Gas mileage was better too. Any thoughts from the peanut gallery???

I use 90wt in my three speed overdrives. From the spillage I got when transporting them from their orginal owners, it looked like 90wt was used by them. I'm not saying that 90wt is the only lubricant to use, just that it seems to work fine for. Also, I can't say that I have had much difficulty in the winter. Although St. Paul is farther north from me (about 10 miles actually).

don't know about synthetic, but I sure wouldn't take a manual trans on the highway that had 30 wt in it!! I think I'd trust the manufacturers recommendations rather than yer friend!!

Bill

Don't know about the 55 with OD but my '49 lubrication manual for the 3 speed without OD states to use 50 wt motor oil.

OK. I was generating a discussion and getting information on lubricating.

So, you have mentioned numbers, and synthetic vs non-synthetic.

Why not 30 wt? Does it sling all over the case and leave nothing for lubricating the gears? Does it get so hot it evaporates, only to reappear when it cools down? I don't believe my friend is cheap or ? He uses 30 wt and changes it every 3,000 miles (when he changes his oil). I like Castro oil. Don't know why, it seems to make a car happy. He is elderly and doesn't drive much. I will probably use this truck but not put on the usual 18,000 to 20,000 miles a year that I need to get to work.

Is 90 wt so heavy that it stays in the bottom, to lubricate the gears? Is that why it is recommended? Do you leave it in the transmission for 50,000 miles (saving time in O & M)? Or is it the choice of 'the book'? Which was written 40 some years ago. They didn't have synthetics back then.

And what about synthetics? In Minnesota it stays thinner. Would that be a problem in Texas? With the price difference of synthetic vs non-synthetic. Are synthetics better? I have so much money tied up in this 'old truck' - I wouldn't worry too much about a few dollars more.

Why do you use what you use?

Again, TIA

Think "GEAR" lubricant. Just my 2 cents.

I couldn't resist jumping in on this one.
I try to use what ever the manufacturer recommends in my truck. After all they have had engineers that are much better than I am at figuring out all the pros and cons of what lube to use. I would not like to second guess the "experts". But I am willing to change if there is good reason.
Synthetics are excellent in many situations but the jury is still out on those. Does the cost out way the benefits?
One big reason to use the recommended lube is the contact between gears is cushioned by the lube. I know we are talking about the transmissin, but how long will your crankshaft last without this type of cushioning? Couldn't this eventually have the same effect on your transmission?
Another aspect to consider is that hypiod gear oil is formulated to prevent air from being whipped into the lube causing foaming. This can lead to early failure of parts that come into contact with each other.
The climate up here does make it hard for the lube to get moving, so a little gentle warm up doesn't hurt.
Always interested in what others have to say. Has anyone used a synthetic for a long period of time and could maybe enlighten the group?

I know this debate could go on as long as what color an engine should be.

When I worked on heavy equipment, Caterpillar would send out service bulletins what seemed like monthly changing what type oil to use in trans and final drives.

Chev uses ATF in T5 transmissions, Honda uses 10w-40, **** in the 1980 anyway used 30w. I think you can use almost anything, only the thinner it is, the more likly it is to leak.

My 2004 Honda CR-V AWD uses 5W-20 in the engine and 10W-40 in the transmission. It's all about tolerances anyway. Use whatever the manufacturer recommends.

In viscousity numbers, 50 weight motor oil is nearly the same viscousity as 90 weight gear oil.
The 90 weight gear lube spec is 13.5 to 24 centistokes at 100C. The 50 weight motor oil spec is 16.3 to 21.9 cSt at 100C.

Stay as close to manufacturer recomendation as possible. I also have a Honda that calls for 30w. But, I have an early 80's chevy 4 speed that calls for Dextron 2 ATF. It has a bunch of miles (over 150,000) and no visible wear inside.

Just because an oil is thin does not mean its lubricating properties are substandard. In fact, John Deere tractors use Hy-Gard oil (which is like 5W-30) in the rear differential/transmission. This oil protects the ring gear and pinion as well as all the plantetary gears in the transmission. It has to be thin to have adequate flow for the hydraulic system, which uses the transmission/differential housing as a sump.

On tractors equipped with a mechanical front wheel drive axle (MFWD) 85W-140 was used in the differential and outboard planetary hubs. However, in cold weather research indicated that the heavy gear oil would be thrown out of the ring gear housing, leaving the spider gears without lubrication which dramatically shortened the life of the differential. Tractors tested using John Deere Hy-Gard oil operating in the same conditions showed no wear. Seal design improved and now John Deere recommends Hy-Gard in the outboard planetary hubs as well.

I feel this issue boils down to seal design more than anything. If your transmission, differential, ect can be fitted with modern seals, I would not hesitate to use thinner high quality oil. However, if you have a gearbox that leaks with gear oil presently, you won't be able to dump in the thinner oil fast enough.

Actually I found something that really helps with the old hard seals. I had a 1963 johndeere that leaked 90 wt like crazy. I knew i was going to have to split the tractor to fix it and needed a temporary fix. I poured a quart of TransX in and every time it leaked down I just added more. In a while It stopped leaking. The stuff is for automatic transmissions to soften seals that are hard. I had a total of about 2 quarts of transx in it. I used it a couple of more years with no leaks.