Good, yes, generally. Then there's better, & best... I put regular in my 740 most of the time, every few tanks I'll go with supreme & it seems a bit snappier when really pushed but otherwise not much difference. I just use it to help keep things clean. The turbo does suffer with regular, especially the '84, no knock sensor on k-jet so it will no doubt blow up if I run regular & drive it hard - even with mid-grade it'll knock under boost. A lot of that is tuning too, though, any Volvo can be set up to run on regular at the expense of some top-end power and economy.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina