Larry, there is almost as much debate about which oil to use as there is about which spark plug to use.
I will provide my opinion, and I stress: IMHO (in my humble opinion).
It is far more important to change your oil and filter regularly (or as required) than it is what viscosity or what brand. Of course, it depends upon whether your tractor is a weekend warrior, a parade queen, or a down and dirty farm tractor. Each one of these uses requires different frequency on oil change, and sometimes the type of oil you use.
I use straight weight, HD30 in the summer, and HD 20 in the winter. My tractors get used to haul firewood, plow snow, and plow/disk a garden in the spring. I also have a couple that go to the tractor shows and parades, but that is about the extent of their workouts. I start them once every 2 weeks in the winter, and run them for at least 20-30 mins.
My oil changes are when the seasons change, and the filter changes each time. soooooo, unless you are farming daily, or using them at competative tractor pulls, just change the oil and filter at appropriate intervals, and use decent oil.......pick a brand.
I happend to use Wolf's head. Penzoil in a pinch.
Hope this helps. PS: I don't have a strong opinion about multivis vs. straight weight. Also, none of my tractors were made after 1951, and most were made before 1946. I also use 1 quart of Lucas for each oil change. May not need it, but it makes me feel better!