I have a 15hp Kohler Pro Command on an Exmark 36" w/b. The mower is working the eleventh season, with at least 6,500 service hours. The initial engine started using oil after the 3,000 hour mark, and I replaced the engine (no rebuilding). I felt that service life was more than I could expect. The second engine remains on the machine, and is used every day. It has more hours than the first engine, but runs very well. It uses just a bit of oil between changes, perhaps a half cup. It starts first pull, and I have never had any carb problems with either engine. In both cases, they have been flawless and I believe I've gotten much more service than one can expect.
I use 10W-30 oil, using Wolf's Head for many years. In the recent past, I've used Shell Rotella 10W-30. For filters, I've used either Kohler OEM filters, or Oregon aftermarket filters. The air filter is serviced regularly, with the cycle depending upon conditions. At a minimum, the foam filter is cleaned weekly, sometimes daily in dirty conditions. The paper cartridge is swapped out at least four times per season.
I have no complaints about this engine, and believe it has provided excellent service for a very long time.