Manual Transmission fluid

[JimT]

So it's about time for the next fluid replacement for my transmission and rear ends. The rears are pretty straight forward gear lube. But the trans has me wondering about different options.

Currently it's filled with the nicely expensive ”mobile delvac 1 synthetic transmission fluid 50”. Only real reason for that is it's what TA used when they did the last drain and fill and so it's what I've been using to top off what leaks out. But it's currently $54/gal or $250-300/pail and with the leak it's really just an experience way to oil coat the underside of the tractor.

The leak is getting worse, but still only requires a top off every 2-3 months (bought a pail 2 years ago and it's just under half empty). I'm wondering if a thicker fluid might help reduce the leak, but not sure what would be a safe option. It's an Eaton Fuller 9-speed converted to 13-speed. In the past I looked up the manufacturer recommendations and they basically said to use what I currently have, but I'm wondering what my fellow real world users suggest?

I operate mainly in New England with temps typically ranging from low teens in the winter to 80s-90s in the summer. Most of my loads are light roofing insulation (about 7-8k on the deck), but sometimes I load up with rolled asphalt (40-45k) or rolled rubber (35-40k). No overweight, usually top out around 65-70k gross, though about 70% of the time I'm grossing high 30's. Half my miles are deadhead. Typically drive around 58-60mph, but when I'm busy I might bump up to about 68mph to stay within drive time limits. Drops the fuel mileage a little, but the extra loads per week more than make up for it.

So what fluids AND brands do you fine folks recommend? I've been using Traveller brand gear lube for engine oil additive. Long winded as always, but I tried to paint a complete picture and hopefully anticipate any questions. And no, I don't personally care whether it's synthetic or dino, just looking to maybe save a little money while maintaining proper performance