The purpose of oil is to provide a viscous film that under extreme pressure does not allow metal-to-metal contact.

Theoretically, the thicker the oil (everything else being equal), the better the protection at high temperatures, where oils thin out the most. Even 50 weight oil is pretty thin at 200 degrees, so the additional friction at operating temperature is probably nit a concern. At low temps, however, the additional friction can actually cause a spun bearing or oil starvation.

In my Alaska experience, I have had a Chevy 350 engine (filled with 10W30 oil) literally unable to be rotated by the starter at 35 below zero when my electric block heater failed to circulate the coolant (the coolant had slushed, so when the external block heater was activated by the timer at 5am, no coolant circulation took place, so the block wasn't warmed). Subaru actually recommented substitution of engine oil with automatic transmission fluid underf cold weather operation (but to limit speed and engine rpm above +10 degrees F).

In real world conditions, we have to assume that the Volvo design engineers tested the components to be sure that the specified lubricant (10W30 in this case) provides prompt lubrication at start-up at moderate temperatures, yet adequate film strength at operating temperature.

This said, if I operated an engine in stop and go traffic in Phoenix in the summer where even good cooling systems can operate well above the rated thermostat temperature (yet below red line), I WOULD be tempted to use a heavier bodied lubricant (say 20W50). The poorer cold pourability of 20W50 would be of little concern since morning temperatures in summertime Phoenix make 20W50 LESS viscous than wiintertime temps with 10W30.

With our engines, however, we have the added problem that these engines are notorious for coking the valves which results in stuck valves that cause pump-up at the least, and valve/piston contact at the worst case.

Therefore in our case, with this particular engine, I would stick with 10W30 synthetic even in hot weather.

Eventhough 0W30 and 5W30 promise 30 weight protection at high temperatures, many car makers specifically advise against using 5W30 in sumertime conditions under high speed or high load conditions. Therefore I would personally avoid these viscosities, even with synthetic, under those operating conditions.

Bob