A 15W/50 IS lower and thinner when hot than a 10W/60.
Castrol edge is not that good, would be much better off with either Silkolene Pro S 50 weight or Mobil 1 from Matt Lewis, at a very good price, 01922 692424.
Depending on your mileage it is most likely that tappets are either knackered or gummed up with varnish.
Either way it might pay to try some valve system lube from Wynns as this will de varnish them as best it can, the actual oil pressure is held in by a steel ball bearing being pushed by a tiny spring against it's seat.
A tapping on first start is fairly normal, and nothing to worry about.
How many miles has it done, new tappets are about £70 from matt lewis, genuine INA ones, easy to change, might as well do belt tensioner at same time too, if a bit old ?
4wd pistons have a slightly revised wrist pin as said, but soem do still suffer when cold, though it is harmless anyway, untill it does it when warm like mine!!
tabetha