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This whole thread has been "interesting" and there are a bunch of assumptions in it about the kind of oil that's already in the gearbox and viscoscity of 'em.

Bear in mind the OP's wagon is a Toyota Tercel, with a low-midrange engine and a lightly loaded geartrain, not a truck or a high performance vehicle.

FWIW, my current Toyota and Nissan (02 and 03 models) BOTH use standard engine oil (5w50) as standard in their manual boxes. I use fully synthetic anyway. This isn't unusual - my ancient CH250 scooter (85 model) also used normal engine oil in the transmission casing.

First step, determine what's there. If "normal" oil, then switching to fully synthetic would give the best gains all round without having to mess around with heaters, intercoolers, etc. As a first step it may be the only step required.

Synchros don't rely much on oil drag - and excess drag will result in heavy shifting. The main reason for high viscosity oil in transmissions was to maintain gear face surface wetting under loadm Most modern synthetics are extremely good at this anyway, which is why manufacturers have shifted away from high-viscosity near-grease in transmissions over the last 20 years.