oil like 10w 40 describes how thick it is in the cold eg the 10 and the 40 when its warm so generally the thinner when cold the better BUT not always asit'll burn off if its thin... generally stick with whatthe manual says for the climate you are in. Run good quality oils or synthetics if ££ allows (it shoudl be noted swapping from mineral to synthetic in high mileage cars is bad)
Ron is the fuels resistence to exploading in the cylinders before it is supposed to. Thats why racing cars/high performance have higher Ron fuels. 95 is standard in the uk - rovers run happily on higher grade stuff but not alot of benefit really except for the additives they put in the more expensive stuff (help keep things clean)
Marginally better MPG using Shell V power and it feels a little more torquey on the bottom end. You can always use a booster additive, but with MEMS it couldn't give a monkeys. Will clean the valves etc though and probably shift a little carbon build up.