I just wait for it to appear in a sale.
Got 5, 10 and 20 weight oils last time (5 years ago maybe) still got loads left.
Got a link to an oil mixing script somewhere too. So I can make anything from 5 to 20 with a slight degree of accuracy.........
And if you think bike oil is expensive, try RC car oil. Nearly 5 quid for a 2 fl Oz bottle.