people say synthetics leaks more but with the 4 cars ive had and run with it ive never had a problem granted zetecs and a tu but i found it makes the engine run alot happier
so the oil stayin mineral oil for the time being till i can affoard to whip the engine out and do the gaskets and fit the new cam 
Yes synthetic oil is good in modern engines but mineral oil is realy what's required in mini engines. They just realy don't like synthetic, the engines were designed to run on mineral and it's realy what they should be run on. Nearly all mini's leak oil and it's best to try and do anything to stop it, there an old saying, "if a mini isn't leaking oil it's got no oil in it" ha ha
Halfords 20w50 classic oil is the best, it has minimal detergants in it and a seal swell additive to stop leaks.
Anyway well done on the work done, I find falkens don't offer as much grip as yokohamas and there not alot more expensive. I ran falkens once, never again yokos all the way,
Still, good luck, carry on with it and get that top hose sorted before it overheats, then you'll have more to worry about than what oil to put in it