"Finally, I remember my manual stating that my engine was designed for 87 octane gas. Today I read a PDF of what I believe to be that same manual, but it calls for numbers that would be Premium grade gas. Which is it?"

What is the "alphabet soup" after the octane number you read in the .pdf? RON, MON, or AKI? There have been contentiously threads here on the subject but the US rating of 87 AKI is sufficient for a factory condition engine.

"Now that I have a source is there any benefit to making the switch in the Volvo?"

Not in my opinion.

"should I stay with 10W30"

Probably unless you live in a temperature extreme that dictates otherwise...say Kuwait...there I would use 20W-50.

I will be honest, these cars have lots of seriously under stressed components where those fluids are. I am lazy and use what is on hand or cheapest for oil, ATF and coolant.

I would look more at the life-cycle-limited components and when they were last replaced.

Since you have an automatic transmission, how old is the radiator? What about the tailshaft bushing and seal (last 50-100K miles on average), center support bearing, u-joints?

How about those front suspension bushings, all your rubber hoses, etc?