Thicker means more drag, slower circulation in tight areas (bearings etc.), and less efficient heat transfer between the metal and the oil. There usually isn't a huge difference between a 40 weight and a 30 weight in those regards, but that's the general idea.
I don't know about supposedly lower flow rate. Oil pumps are positive displacement devices. At any given RPM, pumping thicker oil takes more energy, but the final flow rate through the pump and lubricating system will be largely unchanged. Some of this extra pumping energy no doubt goes into the oil, heating it via viscous shear. Given the flow volume (high) and the heating effect (low), I doubt the temperature difference between a 30 and 40 weight it's more than a degree or two.
It's a loaded question. Lighter stuff transfers heat better. Heavier generates more heat in the shearing and requires fractionally more power to pump. I can hardly maintain the thermostat setting in a heat exchanged environment with 0w-10 in colder weather ..while I never had issues with a 5w-40.
So, raising oil temps can be a "it depends" situation.
To give a little more info this is an LS2 motor with a supercharger. I had a stock LS6 motor previously with the same supercharger and oil temps were right around 190-205 with Mobil1 synthetic 5w-30.
I'm still breaking in the new motor with conventional oil Castrol 10-40 and an AC Delco filter and I'm seeing oil temps around 220 with normal driving manners. My coolant temps haven't changed around 180, but the oil temps have increased around 20 degrees.
So I'm thinking of trying a 5w-30 with a K&N oil filter and see what happens?
The new engine is likely a little tighter, and therefore higher friction, than the used one was. Engines often have a sweet spot for performance and efficiency which kicks in many thousands of miles after they are built.
It would be interesting to know the oil temp. numbers after you switch back to a synthetic 5W-30. I expect them to come down some.
Conventional vs. synthetic will likely play a role, too. I bet you'd see a difference switching to a good synthetic, even if it's 10w-40.
However, it's always a good policy to pick the lightest oil that'll do the job.
Mobil 1 5w-30 is a good pick.
Feel like shelling out a bit more for a bit of an upgrade? Renewable Lubricants (RLI) BioSyn has been said to blow everything else out of the water, and they make a great 5w-30.
Other alternatives are Amsoil ASL 5w-30, Red Line 5w-30, Pennzoil Platinum 5w-30, and Castrol Syntec 0w-30. All of these have very long and very good track records.
All else equal, yes. I'm just saying that there are plenty of 30 weights that hold up just fine, even for very powerful engines. Given that, it's usually of no benefit -- and often counterproductive -- to go for a thicker oil than the engine was made for.