There is nothing detrimental about using the Rotella 15W40 because it will behave like a 40 weight oil when the engine is at operating temperature. Rotella or DELO are the two best choices because both are designed for use in diesel engines.
The Kohler K/M Series engines along with several other L-head cast iron engines from the 50's, 60's, 70's and 80's were DESIGNED to burn small quantities of oil during normal operation. The key issue here is that of OIL LEVEL because it is critical to keeping the crank throw and main bearings heavily lubricated at all times.
Therefore, if you want to use a multi-grade oil, then you must keep a much closer eye on the dipstick level and keep the crankcase topped up at all times. If you forget to do so, then it only takes a few minutes while mowing a ditch or hill on the cross-slope to cause an oil starvation situation and the loud bang you hear will be the con-rod snapping in two.
Kohler themselves WARN owners about increased oil consumption when using multi-grade oils when the ambient temps are warmer than 32F. Very few owners have hour meters installed on their tractors and they end up guessing when it is time to change the oil. With no oil filter, it is essential to the life of the engine to change the oil every 25 hours MAX or sooner if the conditions are more severe than normal.
Oil is cheap compared to the cost of rebuilding at changing it takes a lot less time than rebuilding does.