Your lawn mower is lot different than a 25K race motor, its all about the additives. The recommended test time for most oils is 4~5 years, if you have lots of it, then test it. What happens is the additives separate and the oil does not become effective in performing their jobs. In the airplane world, you get 4 years on a case, after that, junk it. People have been killed because someone got a deal on some old oil and was trying to save $50. When the additives go out of suspension, it wipes the bearings and galls other surfaces because it cant properly protect what need the lube. No you cant put in a can of STP and fix the problem. Some get lucky and are able to use it, others have not. To each their own. Tim