I know you're a RWD guy and although I too own one myself, I find way too much speculation at that board for me.
I'm unaware of ANY fuel or engine related problems to date on any Volvos that are my area and yes, my area's been using reformulated Gas for at least 10 yrs now. My modified 745T remains uneffected by the stuff but I try to use better brands of gas too, like Amoco whenever possible. Old Volvos were known to have cold start problems with high mileage sometimes caused by too much carbon build-up ("gunk") on the backsides of the intake valves and Volvo even had a cure, remove the intake manifold and blast the carbon off with walnut shells. Our shop has one of these devices (still) but in the last 6-8 yrs, it's not been a problem in our area because the gas has been better (more additives).

Now, early 960s and early 850s did have the potential for a valve to burn if the lifter buckets and valves didn't turn as they need to. Either slow rpm driving, "possible" bad gas or lack of oil changes could cause that condition and if the valve doesn't turn, it burns a heart shaped hole at one edge thus loses compression in that cylinder.

IMHO, too many are too quick to blame their problems on "bad gas" when there could be other reasons.

As for old 700s, if you saw some of the (now old) "junkers" we see still coming into our shop, nothing about them surprizes me anymore. Most of those I see are poorly maintained, neglected and barely road worthy these days.