HELP Sludge

[jaybro]

HELP! I changed the oil in my 1999 C-280 today, pumping it out of the pan, put in a new filter after adding 7 quarts of Mobile 1 0W-40. Then buttoned it up. I then checked the level with the dip-stick and removed the oil fill cap to add half a quart. What a shock, sludge all over the cap and down into the valve cover. I have never had a car that showed this. I have used Mobile 1 since buying the car new. It is just shy of 20,000 miles. I thought synthetic oil prevented sludge from forming. When I pumped out the old oil there was no sign of sludge. The previous oil change I had done at my indy, did he use regular oil and not synthetic???
HELP what should I do now, is this common? Should I ignore it?
Thanks for your help in advance. jaybro