My .02 on viscosity: Lawnmower oil (especially that used in tractors)in my opinion ought have a far higher viscosity or weight than an automobiles because as oil gets hotter, it gets way thinner in aircooled engines. As we know common car oil has been 10W-30 (not too long ago). This allows the oil in hot temperatures to go no lower than ten-weight and no thicker in cold temperatures to 30-weight. This would never do in a Harley for example or in my opinion a big V-Twin tractor engine, as the oil would be so thin the bearings would wear out in no time. Tractors needs heavier oil since the engine always runs warm. That's why in my tractor's V-Twin I have run either a 30wt conventional for break-in or a 15W-50 PAO or Diester oil. 10W-30 synthetic's are probably great for a 21" mower.. but beyond that my money is on a 15W-40 or 15W-50 and maybe even a 20W-50. Thanks for taking the time to read this and my experience is based on UAO's that took time and $$$ to prove the point to myself if nobody else. The one time I used a 10W-30 in the V-twin it consumed oil and the UOA was crummy.