Easy job … right? Go to the local auto parts store and purchase a couple of quarts of gear oil. Drain the transmission and refill it with the fresh gear oil. Sounds easy and it is, however, are you sure that the gear oil you bought, with all those new high tech additives, is safe for your old transmission. Well, I am sure that I am not the only one that thought any 85W90 modern gear oil must be OK for my TR3 transmission.

I am in the process of installing an overdrive transmission in my 1960 TR3A. I was thinking of putting synthetic oil in the transmission, but I wasn’t sure if it was a good idea. I started checking the Internet and calling/emailing some mechanics and friends to get some other opinions on the subject. I even called Red Line, a well-known maker of synthetic performance oils and other products, and spoke to an engineer about the use of synthetics in the TR3 overdrive. He recommended their MT-90 synthetic gear oil and assured me that it would give me excellent results. However, a number of British car mechanics said they would not use synthetic gear oil in a overdrive transmission … so I decided it would be safest to fill the tranny with the same stuff we have always used. But, what we always used is not necessarily the same stuff that you find on the shelves of your local auto supply store.

During my investigation on synthetics, someone said that The Roadster Factory’s TR2 & TR3 Glove Box Companion provides information on the correct oil for gearboxes. It seems that the "Hypoy gear oil that is normally available in auto parts stores is known as GL5". TRF goes on to say that they "have learned that GL5 has a limited slip additive that will corrode brass components. All TR2, TR3, TR3A, and TR3B models have synchro cups and brass bushes in the gearboxes, and their diffs may also have some brass or bronze components". TRF says, "Castrol recommends changing to Hypoy gear oil with the "GL4" spec".

OK, so now all I need to do is go to my local auto parts store and buy some GL4 Hypoy gear oil. Well, the first two parts stores I visited only had the GL5 oil. Finally, Pep Boys had one brand with a GL4 rating … a product by Sta-Lube. A quote from the Sta-Lube label says "Not corrosive to copper, bronze or other non-ferrous alloy bearings and bushings". All of the Castrol gear oils I found had the GL5 rating. So there you have it, and by the way, Redline MT-90 synthetic has a GL4 rating, so you could try Red Line in your transmission.

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