The best tranny fluid and lube is the one that gets changed often. I don't think it matters much as long as you change your gear lube REGULARLY. Most people don't do that. I've used synthetics, but the cost is pretty high. If cost isn't an issue, go for it. If so, stick with 90W and change often. As for the tranny, you may want to consider using a high-mileage rated ATF. They contain anti-shutter additives and friction agents that may benefit your tranny. I can attest to this from personal experience. Penzoil is one such ATF, but I see other mfgrs now have them. I know the Penzoil works pretty well.