In any case the coolant temps have to be kept at 195F plus and 220 is better. It's difficult to get the oil warm enough in either Lister or in a two cylinder deere. Unless the oil is warmed up to near 212F. The water and unburned fuel that makes it's way into the crankcase will not be evaported away at low temps. Also a cold running engine drops more unburned fuel and water into the crankcase than a hot engine.
I echo the remarks on ring seating and cold weather drag in gear boxes. My GN burned oil until about 75,000 miles before she broke in the rings. Didn't burn oil for the next 150,000 miles. A quality synthetic SAE90wt at 32F is not as viscous as a mineral Sae 90wt.
As for a "quality" sythetic oil? Most are not made from true synthetic stock but are just reformed mineral oil. Mobil 1, Royal Purple, Lubrication Engineers and Amsoil are the only ones I know of for certain. Lubrcation Engineers will even mix a blend just for your application.
As previously stated, dodge the wide span mutli weights like 5W50 etc. 5W30,10W30 & 15W-40 are about as far as you want to go.
Oil viscosity required depends on the bearing journal dia, width, loading and clearance. In short, a tight sleeve bearing requires thin oil and wide clearances require thicker oil.
The shaft/journal rides on a wedge of oil, not unlike a surfboarder on the ocean. Too narrow clearances and too thick oil = no decent wedge. Too wide clearances and too thin oil = no decent oil wedge. In any case the bearing/journal life is shortened.
Many a small 3600 rpm splash lube engine has went "bang" due to too thick or too thin oil because the operator didn't read or believe the engineers who designed the engine.