right, some answers
Magnatec - IMO its crap but loads of people seem to like it, and its not Castrols flagship oil, its simply a well marketed semi synthetic, there best oil is a fully synthetic called Castrol RS, for non turbo cars the 0W-40 is a good choice, the 10W-60 version is favoured by the 5GTT lads and a lot of Scooby owners
Elf oils, they aint just an oil recommended by Renault cos of payment, Elf oils are GOOD, there developed just like all the others in a lab, and Elf also did, and do, a lot of oil research in formula 1 environments, so they know what there doin, i also reckon there VERY good from personal experience, and a hell of a lot better than Magnatec
Mobil 1, very good stuff and definitely fine for a 172/Cup, its pricey though and if anyone wants i can get Total oil for good prices (Total is same company as Elf) if you have the money to spend its worth it
also as Greeper says Silkolene Pro S is a good oil, ive used the 10W-50 in my car, and the Pro R 15W-50, i was impressed by both, i recommended the Pro S 5W-40 to lots of people on here, about 3 took my advice and all were happy/impressed with its performance
you will find that MOST semi synthetics, and im fairly certain 99%/ALL fully synthetic oils will cling to the engine parts like Magnatec does
lastly, i know no1 has mentioned em, but, additives, dont use em, most contain PTFE which can clog in areas, the ones that dont will simply upset the balance of additives which the oil already has