what oil to use in 1997 vectra 1.8 16v?

can anyone please advise what viscosity oil i should use in my vectra, it has done 155,000 miles and is starting to tap/rattle a bit at the top, i am using castrol magnetec 10/40 at the moment. it is advisable to use a slightly thicker oil or will this do more harm than good? or are there any additive that i could use?