Oil on spark plugs

That is usually either worn piston rings or stuck piston oil control rings or leaking valve guide seals. If the exhaust blows out light blue smoke you can usually tell which it is by when it blows out the light blue smoke. If it smokes almost immediately after starting the engine but goes away as the engine warms up, then it is the piston rings because when cold the piston is smaller, then after it warms up expands and seals better stopping the oil blow by.
If though on the other hand it does not smoke when first started but begins to smoke after the engine has run awhile then it is the valve guide seals, because now engine oil has had time to buildup on top of the head and leak in past worn valve guides or bad seals. A good sign of this also would be leaking valve cover gaskets and if this is found it could also be clogged oil return holes in the top of the head cause again oil will buildup because it cannot drain down to the oil pan quickly enough. These holes can usually be cleaned out with a metal coat hanger opened up.
If it is rings, it might just be stuck oil control rings and sometimes they can be cleaned up with some Gunk motor flush run through the oil for ten minutes at idle, then drain it out, change the filter and add new oil.
This is the Gunk motor flush: http://www.gunk.ca/CAT_MF2C.asp

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