I found this forum and site to be interesting and learned a lot about lubricant and learned how most people misunderstand lubricant. So I registered with my own question. I bought a brand new Gravely ZT XL lawn mower for my half acre lot 2 years ago, that I plan to keep forever and now have 50 hours on it. I want to give it the best lubricants. I not fussy about brand names, not interested in starting the who the best, but whats the best. I have the Hydro Gear ZT-2800. The company doesn't suggest at all using a synthetic oil, but doesn't say you can't either. According to there service manual, they say of course to use 20w50 motor oil. How ever Gravely says, you can use 15w50 synthetic oil in it. In Hydro Gear's service manual, they say this: "The fluids used in Hydro-Gear products have been carefully selected, and only equivalent, or better products should be substituted. Typically, an engine oil with a minimum rating of 9.0 cSt (55 SUS) at 230° F (110° C) and an API classification of SL is recommended. A 20W50 engine oil has been selected for use by the factory and is recommended for normal operating procedures." So lets tear this apart and see what your interpretation is. This would be my interpretation: We all agree that synthetic is better than conventional, so yes, the company is saying that synthetic oil would be ok to use. Thd catch with the minimum rating, is no one rates there oil at that temperature. But considering they recommend 20w50, any --w50 oil would at least pass that rating. Maybe a --w40 would too, but a 30 would be pushing it. Since 5w50 synthetic is more commonly available, would there be anything wrong using that instead? It seems it would be better that it runs thinner when colder. Please no one tell me its too thin, without reading Bob the oil guy's Motor Oil University first. Would there be anything wrong running a 5w30 or 0w30 synthetic oil, instead of a 10w30 oil in the Kawasaki engine? I believe that at least a multi weight oil is better for mower, since it goes through a lot of warm up cool down, then run straight for hours on ends. It takes me about a half an hour to mow. Again please don't tell me its too thin without reading the motor oil university.