I ran Mobil One in my Onan Marquis Gold 7KW on my 2003 Suncruiser. But, I ran 10W-30. 15W-40 might be too thick for that engine. Check your owner's manual and see if that viscosity is approved.
I use Wix filters extensively on many automotive applications, including my Workhorse's engine. But I couldn't find a Wix replacement at that time for the Onan genset. I did buy direct from Onan on-line and their prices were reasonable so I went that way. I do the same with my Fleetguard filters for the Cummins ISL now and haven't had the need to look at Wix, although Wix does make a very good filter. The link to the Onan online store is http://www.cumminsonanstore.com/