They were originally set up to run on about octane 80, which would have been 1 or 2 star I think. Both long since obsolete - 4 star (octane 98) would have been a bit luxury for it but obveosuly wouldn't cause poor running. The compression is sp low (6.7-6.9:1 depending on the head gasket) it'll run on anything remotely resembleing petrol, parafin will probubly work fine with the apropreate carb jets and using the exhaust to preheat it a bit.
I the real modern world standard 95 unleaded is what you want. It is claimed that the valve seats in all the IOE engines, being in the block not the heat, are extremely hard and supereour to unleaded valve seats so unleaded won't harm the exhaust valves like it will in a 2.25 petrol. Other owners are more causous and use adertives. Personly in my dads landy I use an adertive (thats what he wants) but I'm not too fussed if I miss a tank full or what exact quantity I add - Its just a squert of some sort. I don't think this is a bad idea as changing the exhaust seats pritty much requires removing the engine and doing practicly rebuilding it. A pot of redex however is much cheeper, especally as you probubly won't do massive miles in it so it'll last a while.
I time the ignition timeing to standard as stated in the instruction book. It runs fine. I guess I really should ajust it to take advantage of the far higher quality fuel but rover only seemed bothered in telling you how to ajust it on even lower quality fuel - as I said it runs fine so I leave it be. Its not like ajusting it will turn it into a Fararri.
