Inadequate spring pressure will/can cause slippage..... put the oil thing aside for now. If the plates are worn enough, the total stack of plates can be insufficient length, thus the clutch springs can't provide enough pressure. Basically saying, measure the friction plates for thickness, and you can calculate if the total plate pack is thick enough to the minimum, or how much you have lost over time.... ST11 spec for total stack of 10 discs + 9 plates is 54.72-55.72mm, service limit 54.2. Could also check clutch spring free length, service limit is 40mm.
Check also the outer clutch boss for notching where the plates have been wearing against it. If there are definite notches, depending on how bad they are, if any, this can cause a plate to "stick" and not compress by spring pressure..... depending on severity, these can be filed smooth.
However, do this first..... Check that brass bushing in the clutch lever that the master cylinder push rod fits into. If worn oval, scored or even possibly punched through, you'll lose master cylinder stroke or it may hang up..... check and grease annually. Replace if worn, you may be surprised how much affect this bushing can have.... most bikes take the same one, so available many places.
Edited with ST11 specs.
