I know Vauxhall blamed some of their engine problems (sticking valves and burning of valve seats) on 'supermarket fuels' a few years ago - Don't seem to have a problem with supermarket fuel in my Jazz, but would rather not use it anyway. The Civic that I had before definitely ran better on Total or BP and very well on BP Ultimate, but got too expensive to keep using it.
I normally add molyslip to my engine oil gearbox oil and change engine oil at least twice a year irrespective of mileage, once at beginning of summer and again in the autumn - have never had a problem with any car I've had (and I normally cover 20K+ miles per year) with burning or leaking oil and definitely no 'sludge' problems.
Molyslip is 'molybdenum disulphide' and the 'disulphide' bit is attracted to metal and sticks the ultra-slippy 'molybdenum' plates to the metal parts of the engine bearings. With the sticky 'disulphide' bit close to the metal bearing surfaces and the slippy 'molybdenum' plates on the outside it reduces friction in the engine and provides protection during the dodgy period during startup / warmup when the bearings are moving but the oil hasn't been pumped round to them yet (metal to metal contact). Up to 80% of engine wear takes place when the engine is cold - that's why 'motorway miles' are no worry to the engine, it is hot and the oil is flowing and all the gunky deposits get burnt off and blown away.
Don't judge a car purely on mileage - with modern oils, high mileage is not a problem - however avoid older low mileage vehicles, they may look a bargain, but be aware that they have probably had lots of cold starts and short journies - causing high engine wear. The kindest thing you can do for a modern car engine is to cover at least 25 miles a day, this keeps everything in top working order, including the battery and cat-converter.