i have some reasonably solid evidence that thicker or thinner oils, as well as dino and synthetic, do not change the operating tempature to any measurable degree.
i have a 5hp honda gc160 ohc pressure cleaner and i use it frequently for semi commercal purposes. while i fully realise that this engine is not an automotive engine, it is aircooled and is at the mercy of the elements, so there is no thermostat to regulate the cooling tempature. the engine just runs as hot as it gets. i also run the pressure cleaner under a steady state at around 3400 rpm. i awalys run it in the same manner so the results are reproducable.
i also have an infrafred digital tempature gun that i use to monitor the hondas oil tempature as well as cylinder tempature.
i have tried various oils in the engine, from 15w50 diesel oils wuch as rotella and delo, to synthetics like german castrol and mobil1 to dino car oils like supertech 10w30, gtx sae40, gtx 20w50, and some blends of leftover oils put together into random mixes.
what i found basically is that all of these oils make the engine operate at the same tempature. there is no difference in operating tempature on the cylinder head, or the oil drain plug between any of the oils. which is where i normally point the little laser beam that comes off my tempature gun. the oil drain plug is 1 spot, and one of the cylinder head fins is another spot.
actually, running the pressure cleaner in the shade lowers its temp by about 15 degrees. running it when its 70 degrees outside lowers its temp by 10 degrees compared to running it when its 80 outside.
so taking into account the difference between shade and ambient temp, the oils do not change its operating temp. which seems to be around 180 at the drain plug, and 240 on the intake side of the cylinder head when its 85 outside. it gets to almost 300 on the exhaust side at an 85 ambient temp.
maybe in theoretical labratory conditions a thinner oil or a synthetic might have more potential to transfer more heat from the piston to the cylinder, or to the block, but in real world conditions, the results are so small that i was not able to measure them.
i have also casually tried this with my cars and got the same results as the pressure cleaner. weither its dino, synthtic, thick or thin, the oil drain plug is at roughly the same tempature. the ambient temp and weither or not the car is in the sun or shade is the only thing that really makes a difference.