Both of my Kubotas and both of my Kohlers are running OEM oil at the moment, and my Champion generator has a mix of Kohler 10w30 and castrol gtx 10w30. They will all be running some flavor of Rotella on their next oil change though. But I might use mobil 1 15w50 for the 35hp Kohler, as it's around the same price point. All fairly new, so I went with OEM oil and filters for the first couple oil changes in case they had some issues while still under warranty. As it turns out, my M7060 has a problem with 2nd gear, so hopefully Kubota will play nice when it goes to the shop.
If you want to stay in spec with grade but are worried about viscosity (I know most Kohler engines only spec 10w30 for most operating temperatures), Rotella T5 10w30 is pretty darn thick for a 30 grade. It's 12.2 cSt @ 100C and it meets ACEA E9, which means it has to have a HTHS viscosity greater than 3.5 cSt @ 150C. Most 10w30s are just a bit over 10 cSt @ 100C. For instance, Amsoil ASE 10w30 Small Engine Oil is 10.3 cSt @ 100C and 3.3 cSt @ 150C, and Mobile 1 10w30 is 10.1 cSt @ 100C and 3.0 cSt @ 150C. Rotella T6 5w40 is a great synthetic HDEO, that many have said even picky Kohlers with hydraulic lifters like. T5 and T6 are both pretty inexpensive at Walmart (15.97/gal and 19.97/gal). They don't carry any gas engine API certs though, so using them in some motors still under warranty may not be the best move.
On the topic of Kohlers with picky lifters - My 35hp Command Pro on my chipper/shredder calls for 10w30 up to 80F and then 20w50 for hotter temperatures, with no mention of an Xw-40. Pretty odd. I can also testify that this motor doesn't have any lifter tick using 20w50.
I'm sure the OEM small engine oils are good choices as well, and still somewhat reasonably priced (Kohler 10w30 is 5.99/qt @ TSC). Kohler claims to have a stout add package with lots of zinc and moly, stay in grade stability, high TBN, etc. It looks like a true HDEO, spec'd CG-4 for older/small diesels, as well. I haven't found a VOA on any Kohler oils though, so who really knows. The product sheet looks good though.
https://resources.kohler.com/power/kohler/enginesUS/pdf/E2265 Oil_SellSheet 9854.pdf The PQIA analysis for Kubota 15w40 looks pretty good too. There's no HTHS or NOACK data though.
http://pqiadata.org/Kubota_15W40.html