I have two Exmark mowers with Briggs Vanguard 35hp engines. I am trying to get as much life out of the engines as possible and was wondering if I could use a fully synthetic 10W30 oil. One engine has 416 hours on it and the other has 200 hours on it. Briggs recommends SAE30 oil for them and I've been using Shell SAE30 dino oil with 50 hour interval changes. My concern is that in this part of the country in the summer it is really hot and humid. My mowers are run hard seven to eight hours a day in the heat so I'm looking the most protection for these air cooled engines. My main question is will 10W30 synthetic be okay when SAE30 is recommended? Is 10W30 thinner than SAE30 and is there such a thing as straight thirty weight synthetic? I don't know a lot about the difference in viscosities so any help would be appreciated. Also do you think running synthetic will prolong the life of the engine? If I go synthetic can I increase my current 50 hour change interval to 60 or 75?.