I'm not really sure why this was your conclusion.
Here's what I got out of ther various post in responses as they relate to your specific engine and use. (This plus having read Widman, Haas, and number of other relevant aricles on oil).
1. The higher temperatures you observed are typical result of filling a block.
2. The temperatures and pressures observed are within acceptable range for continuous use (although I don't recall exactly what you posted at Moparts about top rpm and pressure, but IIRC the concensus was it was OK).
3. If you want to match the viscosity to be the same as it was before the block was filled, you can use a heavier wt oil or install an oil cooler.
Also II'm also not sure why you are asking such a broad question. First, a normal street driven production motor with a flat tappet cam can use some modern oils that have good modern antiwear packages without zddp. You want to figure out which ones those are, especialy as the big manufacturers, revise the products and marketing people revise the labels every 6 months? The big companies are not terribly interested in the small market. However they do have some market share that needs products that demand high temp, high resistance to sliding wear, similar to older flat tappet needs - eg european turbo deisel truck. And after all that, my impression is that the spring pressures on your motor are much higher than typical production vehicles used before the roller took over. That's why most of us stay with the tried and true. A very few folks can work with tribologists who really understand the addative packages and all the other characteristics, most of us can't afford that level of service which is normally for industry an big money race teams.
Second, as Widman and others explain, the term 'synthetic' can be very broadly used by the marketing people. So you can't just ask about 'synthetic' and expect a useful answer.
That said, in terms of physics, I can only think of two explanations as to any oil would result in lower temperatures. If the viscosity is actually better matched to the engine needs so more volume is being pumped through (instead of dumping out the releif valve). This would result in more heat being carried off. The other is either due to viscosity or additive package, there is less friction and therefore less heat. Hard to imagine that alone is significant.