Hello Old Farmer,
The 80W-90 will actually have a better low temperature start-up than what the origional SAE90 would have. The warnings regarding the usage of EP's was lifetd by AGMA a few years ago, assuming certain conditions were met ie less than 90 degrees C. This of course assumes that you are using an oil from a recognised OEM blender. If your winter ambient temperature conditions are less than -10C or so then you might want to consider something like a 75W-80 gear oil (~47 cSt @ 40). Or if you think that you just need a lower start-up product then you could consider a 75W-90 product. These are avaliable from most of the major NA suppliers.
An engine oil like 15W-40 has a vis @40 of around 110 cSt @40. Again if you have start-up temperature concern issues you could select a 0W-40 product this would lower the pour point to very low temperatures -35C (you probably wouldn't want to be operating yout tractor at these temperatures)
hope that helps