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That 40W will be awfully thick in the winter - especially at start up. The 20W50 is thinner then 40W at start up but thicker when it reaches operating temp. If your tractor was running hot or getting run hard 40W would probably be better - especially in the summer months, but if all you are doing is running it for light duty stick with the 20W50. Polymers are added to a light base(5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot. Always use a multi grade with the narrowest span of viscosity that is appropriate for the temperatures you are going to encounter. In the winter base your decision on the lowest temperature you will encounter, in the summer, the highest temperature you expect. The polymers can shear and burn forming deposits that can cause ring sticking and other problems. 10W-40 and 5W-30 require a lot of polymers(synthetics excluded) to achieve that range. This has caused problems in diesel engines, but fewer polymers are better for all engines. The wide viscosity range oils, in general, are more prone to viscosity and thermal breakdown due to the high polymer content. In the end using straight 40W will give more thermal protection then using 20W50 under high temps, but the question is, are you using the thicker oil for thermal protection or to bridge the "tolerances" that wear has added to your engine?
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