Well now that everyone has given you the "Skinny" on the oil... There was a few other things that you may want to put on your list of "To Do's" when you check the mower again.
1. Were the Muffler bearings greased? If not, the muffler will start to squeak after about two seasons...
2. Were the tires rotated? If not, then you could start making offer centered cuts resulting in brown grass from being cut twice in one mowing...
3. Was the mower blade rewound? If it was not rewound, then it could start to lose it's hitting power and the grass will have "Patches" of grass standing after a good afternoon of mowing...
4. If it was a riding mower, did you have the Halogen fluid levels checked in the lights? No brainer there, no fluid, no lights, or at best, very dim...
5. More than likely, it was an air cooled engine, if so, then did you check the air accumulation tank that stores the cool air? If not, then the engine may have sharp periods of overheating...
Well there may be some others than can give a few more pointers, but that should be a bit of help...