Oil On Spark Plug Threads

[E150GT]

Hey all,

two days ago I changed my spark plugs in my 420sel with berus from the stealership. I had changed them out in demcember but I used platinum.
When I changed them out the first time I saw just old used sparkplugs. This meaning that they looked proper with no signs of trouble as far as excessive fouling or detonation etc. Well this time around three of the eight 2 month old plugs has small amounts of oil on the threads. It didnt look like the oil was burning but was still present. I dont know what this indicates as Ive never seen this on any of my cars which have included two 200,000 mile + fords. Also at the same time these plugs were put in I did an oil change using mobil delvac 15w40 as apposed to 20w50 regular oil. Could it be an oil thing or a more serious issue? I havent noticed any oil loss since I changed the oil in december.