5/30 doesnt have to cost more, but its a thinner oil so would reduce the tappet noise, however if your saying its losing oil then if you put 5/30 oil in then your gonna probably lose it faster. if your handy with a spanner you could remove them and leave them to soak overnight in a bowl of 3in1 oil.
are you sure its the tappets? are you getting blue smoke from the exhaust?