This will be moved to the 'other bikes' section, but as long as you're not using oil with 'friction modifiers' or says 'fuel saving' on the label...oil is not the issue.
How many miles are on that clutch?
is it simply worn out?
Old clutch springs sometimes become weak, if you replace the clutch discs, replace the springs too.
Is the clutch simply out of adjustment?
A very old clutch cable may have stretched over time making a correct adjustment impossible without a new cable.
Think it through logically, there are a few different factors involved.