Can my volvo turbo XC70 2000 handle non-premium gas?

The manual tells me to use 91 octane or more, but what harm am I doing if I use regular gas?

Follow the manual. If it says “Premium is REQUIRED,” then it’s requred. If the manual says “recommended,” you MIGHT be OK. Might be. I wouldn’t chance it, personally.

Over the long term using regular can damage the engine. The computer will adjust things and mask the damage so you won’t notice it, but it will happen nonetheless. In the meantime your fuel mileage will probably decline slightly, and the car won’t perform up to its full potential. If the mileage decreases, what are you really saving?

If you don’t want to pay for premium, why did you buy a vehicle that requires premium?

Normally, my opinion is that it depends. All cars and situations are different. Most will not suffer; some will.
The problem with yours is the turbocharger. One must be very careful about low octane fuel in a forced induction engine because pre-ignition can be death to a turbocharged engine much faster than a normally aspirated one.

The only way I would even consider it would be IF there was zero rattling, no drop off in mileage or performance, the EGR system is operational, and a careful examination of a new set of spark plugs after a hundred miles gave it a clean bill of health.
If you’re not sure about this stuff then best stick with the 91.

It Volvo (owner’s manual) says it is required. then you risk damage not using it, if it is recommended it may reduce your mileage and power. Using anything higher is a waste of money, assuming the car is in good condition.

Maybe, but I wouldn’t try it. It’s too late for your wallet to have indigestion. I run regular in the Yaris. You didn’t want one of those and I can see why.