I found my notes from about five years ago when I first went on a hunt for 90W non-mineral gear oil and couldn't find any at my local auto parts stores. I was changing the pumpkin in my Eaton from a 4.56 to a 4.10 at the time, so I knew that I'd need the right stuff to refill it.
I called Mercury Marine to check on the Quicksilver 90W. It is not synthetic, but it does have synthetic additives. It is made for higher horsepower marine lower units with hypoid gears. The synthetic additives are introduced to improve the load limit capability of the oil. It does not have anything in it that will hurt brass parts. It is NOT to be mixed with 80W-90 lower unit lube. The 90W is designed for lower units that will have more of a load on them. The only reason that I thought about this product is that I remembered my marina used to buy it by the 55 gallon drum.
I found this thread interesting, so I did a little more research. Automotive gear oils have a GL rating; marine gear oils are not required to. It turns out 90W gear oils are not all the same. Some are mineral, some are not, some are synthetic, some have synthetic additives, some can hurt yellow metals. Even within categories there are different GL ratings. GL might stand for 'gear load' because the higher the rating, the higher the rated gear load. Here is a link to a quick read that others might find interesting--
https://en.wikipedia.org/wiki/Gear_oil There is lots more info readily available on the internet if you want more detail.
It seems that the best solution for both trannies and hypoid rear ends would be a non-mineral SAE90 gear oil with a GL-4 rating. GL-5 would have a higher load rating, but the additives used to achieve that load rating can be detrimental to yellow metals, so GL-4 seems to be the ticket. Gear oils rated below GL-3 are not listed for hypoid gear applications.
I found a 90W non-mineral, non-synthetic GL-4 gear oil made in the USA and marketed under the name Lube King. It is priced right at $29 delivered for a two gallon jug, not too much more than the Tractor Supply mineral oil. I bought a jug. This is what I plan to use going forward in my SM420 and Eaton rear end.
Getting back to the Quicksilver product, it is probably comparable to an automotive gear oil rated as a GL-4 based on the description, but that is simply an interpretation on my part. It does not have a GL rating. I will start using the less expensive Luke King product. I'd use the Quicksilver again if I couldn't find the less expensive alternative. My local Walmart has the Quicksilver for $13 per quart. That's $104 for two gallons compared to $29 for the Lube King product. Sounds worse than it is because it only takes about 3 1/2 quarts, as I recall, to change out both the rear end and the tranny.
I also found a comparable Citgo product, but it was only available in 5 gallon pails. NAPA also has a couple options that you can find if you type SAE90 into their search engine, but they are also only available in 5 gallon pails...and they carry a 5 gallon pail of SAE90 mineral oil.
I have found this to be a very interesting thread and thank all who have contributed,
Matt
