My question is:
A) What temperature should oil reach on a cold day before hitting the road?
B) What temperature should oil reach before WOT and high RPM is safe?
C) How hot is too hot?
A) Just a couple of minutes of idle on a cold day sufficient to make sure oil has gotten to all parts of the engine is OK.
B) 150F is a good temperature at which most oils have thinned out to be within 10-15% of their KV100 viscosity.
C) The HTHS rating test is done at 300F, so consider that a short-term temperature where the oil still has sufficient film thickness to protect your engine. But continuous operation a 300F+ leads to higher oxidation of the oil. Over 280F is where I would consider getting a better oil cooler.
Oil has always been Subaru OEM 5W30 synthetic. I don't remember what group it is, but I do remember it's mixed for Subaru by a smaller company.
On the last oil service, the dealer switched to Mobil 1 5w30 synthetic, which has a terrible rep for killing bearings.
I think whatever killed the first block is what caused the sudden catastrophic oil loss.
Both the Subaru Idemitsu 5W-30 and regular Mobil1 5W-30 are thinner Resource Conserving oils. There have even been failures on Subaru filled WRX's serviced at dealers as well.
Whatever oil you decide on, do not use any Resource Conserving GF5 oil.