15w/40 mobil 1 mate, worked for me, but mine kept burnin it (piston ings)

My mate has the same year corsa gsi as me (L 94)and it runs about as smooth as mine apart from in the morning. If i start mine alongside his, his sounds like a totally different engine because it ticks so loud!! When its cold i.e winter, it sounds like a tappet is sticking. When you rev it the ticking goes faster and sounds terrible but its so easy to get rid of it. If he turns the engine off and starts it again its goes or if he revs it hard it goes. The loacal garage says that its a sticking tappet and to use thicker oil. They also say dont bother fixing or adjusting it because it does hardly any damage (the garage are very honest and friendly and never rip us off) What do you guys reckon? What oil is best for a gsi to keep it quiet and looked after? any help would be very appreciated!!!