What causes oil to collect in the spark plugs

the spark plug tube seals in the valve covers are bad(hard,brittle).when they go bad over time they will leak and fill the spark plug tube wells with oil.you need to buy a valve cover gasket set.even though you didn't list the engine size in your question,it doesn't matter because the 3.0L and the 3.5L takes the same gasket set,Felpro/Valve Cover Gasket Set> Price: $43.99> Part Number: VS50461R> Warranty: 1 YR> Notes: With spark plug tube seals; Permadry molded rubber gasket included,this price is at Autozone..you also need to check you spark plug wires.if they are mushy where they go down inside the valve cover and connect to plugs,replace them also,if not just clean em up good.mark all the wires before unhooking,so you know where to hook em back up.also get ALL the oil out if you are planning to replace the plugs.you don't want the oil draining down on pistons,even if not changing the plugs,get all the oil out of each well! if a shop does this,they will charge you $400.00-$500.00 for this! do it yourself and save $$$$!