Bryce, 20w50 would be on the thick side but will probably work. I suspect that your 3ph will be very slow until it warms up
MyTE20 was a work horse. Over the approx 16 yrs I worked it I had to replace the waterpump, oil pan (unusual story) and rebuild the oil pump.
The waterpump was my fault. I was hauling some big rocks w/a pond scoop from the garden to my dump spot when I twisted the tractor alittle too much in a little ravine which caused the fan to get up into the shroud, she leaked after that.
I bent the oil pan right at the point where the oil filter plate was so it started leaking oil. How I bent it was when I was brush hogging a corn field one of the front wheels rode up over a bump and came down hard right over a cutdown corn stalk. That ole cut down corn stalk was just high enough and stout enough to bend that steel oil pan, unusual I rekon
I rebuilt the oil pump w/ a kit (new gears and a gasket) that increased my all around oil pressure by 10 psi!
Surprisingly all the parts were readily available from the same old timer that told me about the kerosene trick

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Another trick I used was to use a good gasket sealer on both sides of the rubber gasket that seals the oil filter plate to the oil pan. Never had a leak after an oil filter change.
I also had to seal a couple seams on the gas tank. I used JB Weld after I cleaned the spots up real good down to the shiney metal and they never leaked.
One more. The std rear seals were notorious to leak oil/grease down onto the brakes making them useless. I found a kit called "Sure Seals" that stopped the leaking. They are made with a steel body that fits perfectly into the axle housing and they have a tight fit rubber automotive type seal that seals around the axle. If I remember correctly you leave the std seals in there. After I had the brake shoes relined my brakes worked great. I found those turning brakes to be very handy

You have to remove the wheel, brakes and backing plate then slide the axle out. Litely pound the sure seals in then reassemble. Works good.
My TE20 was still 6v and I kept it that way. How I got it to start easier would be to use one of those turkey cooker injectors, fill it with alittle gas then directly inject it into the carb. I had a rubber radiator hose between the carb and air cleaner so pulling it off a little bit at the carb wasn't any problem and it would start right on up, even when freezing

Course most folks around my way change there TE's, TO's and Fords over to 12 volt but my little trick worked good enough
I started missing that tractor as I watched it's new happy owner turning out of my driveway with it on his trailer......
In any case, let us know how it goes.
Dave