use 15/50w oil

u will probs find that the stem seals have started or are perished also worn valve guides will increase oil usage u should also see signs of oil on the spark plugs

If it where me i'd have the head removed fully checked and rebuilt as it is now 16yrs old and will have worn parts in it and get them replaced

valve guides if worn
stem seals (all of them)
new camshaft bearing fitted
head de-choked and all valves lapped back in
and a very mild head skim if needed