What sort of oil have you been runnig?
Any old thing IMHO modern oils are so good that you'd have to work hard to find an oil that wouldn't be good enough for a DL (old design) engine
The dealer usually puts Motul in - since it's been out of warantee I take it to them ~ yearly, usually beginning of winter. (Good dealer, good mechanics and a few times they've spotted problems I've missed). Usually gets another change in spring - and I put (non-EC) car oil in then. Hot as hell here summer and the thinner bike specific oils don't last as well.
The springs were definitely below spec. for length when I checked them - so it probably wasn't the oil causing slip.
The clutch will last a hell of a long time for most riders - I'm assuming that the plates were max thickness to start with, but if they weren't there was a lot more plate life left than my estimate.
The springs will be marginal well before the plates go is the main thing to remember. The slip isn't bad, but I really noticed the extra drive powering out of corners with the fresh clutch.
I don't think you'll EVER have to change the flat steel plates - but I got a good deal on a full kit so it was only $10 more to change them all. You are up for a new gasket oil and coolant just opening it up - that's a sizeable fraction of the total cost.
I should also have replaced the thrust bearing. Forgot to buy one, but the old one looked and felt 100% O.K. anyway.