In my CTSW, I had the prop set for 5400 WOT in level flight. That improved my oil temps from the previous, coarser, prop setting but I was still running warmer than I wanted. About mid-summer I set it to 5550 WOT and it runs cooler. Now I can go from near SL to 10,000' just barely getting into the yellow on fairly warm days. This prop setting equates to about 5100 in an 80K climb.
So, my experience, and that of other CT owners, is that a little higher WOT RPM in level flight nets cooler oil temps. Try it. You can always put it back if you don't like it.
My Rotax experience is with the CT so I can't speak to the -12 installation but, yes, these engines can run a little warm on hot days. I would not be concerned about running in the mid yellow in a climb but if you are staying in the yellow in a 5200 RPM cruise I would be looking to change something. You say you are cruising at 5300 and that will give higher oil temp than 5200 but at 70 degrees that does seem a little warm.