Interesting idea. I don’t think a higher viscosity will reduce oil temps, and I don’t think increasing the viscosity will reduce the aging/ deterioration /breakdown negative effects of high heat. It May increase film thickness.
In fact, a thicker oil will have slower flow thru bearings, which will increase oil heating. At the same time, viscous heating (as well as parasitic losses) will increase. Lots of anecdotal evidence that changing to a thicker oil actually raises measured oil temps. Higher oil temps could mean better heat transfer at the exchanger, but lower fluid flow rates could reduce heat transfer at the bearing. In fact, the data I’ve seen shows that higher flow of lower viscosity oil mores more heat...but likely very specific to each design/application.
If the mission was to increase film thickness, I’d agree. But I believe film thickness is adequate (one of the few things I trust they got right after all these years) as long as the oil maintains integrity.
But, yes, definitely a 1st-world problem