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Is there any benifit or disadvantages to putting Synthetic gear oil in my stock 54 Chevy pickup 3 speed & rear?

according to several gearbox manufacturers. (industrial machinery gearboxes) they say if you use synthetic oil you can withstand 25% more torque. Better fuel economy, although that is hardly an issue. that seems enough for me...disadvantage would be the extra $20 you would spend on it. Personally I don't care about $20.

B52Gunner, it is very beneficial to the manufacturer, wholesaler and retailer as the stuff is very expensive, thus more profit.

The disadvantage is to your wallet.

The use of synthetic lubricants in non-race applications is a triumph of marketing over lubrication science. The best current example of that marketing skill is the ad showing the youngsters in the parts store buying Castrol synthetic motor oil while the adman tells us that Castrol synthetic is 8 times better than Mobil synthetic. Well, hello! How good does it have to be to be adequate? We're not going to the moon, we're doing an oil change on a daily driver.

You will get many opinions but I don't use it in any of mine. The only advantage would be in the trans it will shift easier in real cold weather.

I like 80w140 synthetic in rear ends. I use 50 wt synthetic gear oil in my transmissions. However, many consider it too slick for synchronizers to work properly. The transmissions were built for 90 w straight mineral gear oil and they ran a lot of miles with that. If you are willing to spend the money and really want a synthetic in your transmission, this stuff is good. http://www.redlineoil.com/product.aspx?pid=46&pcid=7 You will probably hear as many different opinions as people you ask.

actually the important thing is having oil in the transmission, people use every thing from ATF, to 140wt. with the only real consequence being the transmission works...

Having the CORRECT oil in the transmission is more important. Using the wrong fluid can dastroy the transmission. Using grade/type the manufacturer requires, be it ATF, gear oil, motor oil (yes, some call for motor oil) is always the best policy.

the CORRECT oil is some engineers opinion, they specify all the aforementioned oils with the same result, the transmission works. although I agree that some form of gear specific oil should be used as there is extreme pressure additives to deal with the high shear loads encountered in a gear box. However that doesn't change the fact some engineer has specified any of the above oils at one time or another. Using a synthetic will allow a lighter oil to be used due to the superior lubricating qualities giving better efficiency. But in this application whatever benefits gained with synthetic is probably moot, as there is so very little stress generated with these engines, and if you are after fuel economy you are driving the wrong vehicle. Personally I like synthetic. (yet another opinion) So to answer the original question, if you want to use synthetic and aren't concerned about the extra cost, go ahead, the only disadvantage is the above mentioned extra cost, in all other respects it is a superior product.

You have to be careful what you put in these old trucks. The trans. are not sealed good. Thin stuff will leak out &/or run back into the rear end. The rear end must have GL-5 rated grease. If the seals have not been replaced in it lately too thin of grease will leak all over the brakes.

But if a transmission never specified ATF, for example, as the correct fluid, filling with ATF can/will cause premature failure. The engineers don't make specifications based on "opinions", they do lots of testing to determine what works best. As far as using standard or synthetic, that can even come in to play, as many later model transmission call for synthetic only (like the NV4500), and use of regular gear oil will ruin them.
However, with a trans that originally specified a conventional oil that has a synthetic equivalent, then there's no issue with using the synthetic, other than making sure the seals and gaskets are in good order to prevent leakage.

Thanks everyone. I only drive my truck about 3,000 miles a year. I would spend the additional $20 if there was any significate value. Based on the comments, if I understand them correctly, there is no major reason to change to syn. Everything is working fine... so why mess with it. I'll just keep my gear oils topped off & clean.

oh they definitely base much on opinions, I have gone around and around with engineers. if you hold so much stock in engineering, explain the vega and the pinto