For all intents and purposes, an Xw30 is a 30 weight oil. 0w30, 5w30, and 10w30 are all 30 weights at operating temperature. Don't stress about finding a SAE30, just buy some 10W30 Mobil to keep the same brand.
The newer Kohlers DO have recommendations on synthetics, and they recommend the same weights and drain intervals as the conventionals. Interesting enough, they recommend anything from a 5w30 to a 15w40 (depending on the piece of equipment).
I have read that the newer automotive oils have been reformulated for the newer cars with roller cams...they supposedly do not protect flat tappets as well as the previous formulations.
Personally, I used 15w40 Rotella in my K-series Kohlers for a long time with good results...I now have switched everything over to 5w40 Schaeffers, mainly to have a single oil on the shelf (of which I have almost 20 gallons sitting there!!).
The main thing with OPE is to actually change the oil!