I was cleaning out the shed by my grandmaws recently an decided to get the old Briggs push mower running that has been sitting for about 10 years, its around 15 years old. Its a flathead 3.5 or 4 hp, I drained the old gas, put a new diaphragm on the carb (carb mounts to tank) new air filter as the old one was dry rotted and a new spark plug and she runs like a champ. I do have a few questions. I drained the old oil and filled it with some Rotella 15W40, it holds half a quart, would it be safer to go with some SAE 30 due to concerns of viscosity breakdown or will the 15W40 suffice? Also, its a throttle free design, pull the lever and start it but If I tie a rope on the handle for it to stay running I can go by the throttle lever and rev it up by hand. Am I missing a spring that allows it to stay at a constant speed? Would Champion RJ19LM be the right spark plug? Thanks for any info. My grandmaw is glad that its running again. I offered to cut her grass but she refuses as she wants to do it, at 76 years old if that makes her happy I'm not arguing with her.