[JohnM 855 T5R]

I always use Shell V-Power super unleaded in my 855 T-5R, C30 T5 Polestar and C70 T5 Polestar (unless I'm stuck in the wilderness with no Shell station in range).
Super unleaded is factory recommended for all 3 cars with an octane rating of 98. Many super unleaded brands come in at only 97 Octane (the legal minimum). Esso Synergy+, Tesco Momentum and Shell V-Power come in at 99.
On the higher performance Volvo's it certainly matters because the higher octane rating delays the onset of pre-ignition (or knocking). If the knock sensors on the engine detect knocking, the ECU will back off the ignition timing reducing power. I remember reading about a back to back rolling road test done some years ago on a Volvo V70R using a regular 95 octane unleaded against a 98 octane super unleaded. The difference was 19 BHP, presumably due to the action of the knock sensors/ECU! So in my view, always go with 99 Octane. I use Shell because its been shown by independent lab tests to have the most effective additive package for keeping the engine internals clean.

John