I have use synthedic oil only in a few of my engines. I used it on my 1950 demo cub after I rebuilt the engine some 10 years ago. I first ran the engine long enough to seat in the rings with penz oil straight HD 30. Then I used royal purple 20-50.. I havent change the oil since but it only gets a few hours of running time per year. I can go out at - 10, the demo will fire right up. I also have a IH model 3200 skid steer with a wisconsin VH-4D engine. I totally rebuilt the engine 5 years ago. I ran it about 100 hrs on penzoil HD 30. In the summer there was a lot of heat from the air cooled engine. I draine the 30 wt penz out then used royal purple 20-50. I noticed a drop in temp coming from the cooling system, How much? well I can only say the cooling system blows out on your back. I couldnt run it for only a couple hours before. With the 20-50 the air coming off the engine is only warm not hot. It has started up at -15 with no problem. Now I would not run synthedic in any engine that has old leather seals. The synthedic will disolve the seals. I have been told that it is impossible by people some years ago on this forum. But I have experionced it first hand with engine seals and any other seal in the lubercating system that is leather.. I also have wroking cubs that have penzoil HD 30 in them year round. Cold weather starting hasnt been a issue. Oil pressure comes righ up BUt I do let them warm up in cold weather.
Collector of Farmall cubs and cub cadets.Injoy helping people keep their cubs running. Years of experipnce.