Bought the car in a no start condition.
The Catera, as most cars these days, is too complex for "my mechanic" or "my guy" to fix, and if the dealer you take it to can't figure it out, don't pay them, that's their job to find the source of the problems.
I am a world class certified Cadillac tech. Any tech that has worked on a Catera LX can tell you the heater control valves leak, and fix it for about $350.00 AT THE DEALER before you run the car out of coolant and blow the head gaskets - $4500.00.
And the timing belts, pulleys and tensioners are covered under a recall for free on most all Cateras. Also a water pump alone is only about $300.00 at THE DEALER, so if some shade tree is charging $800.00-$900.00, you are getting taken.
The mayo look on the oil filler cap is normal, not an indication of a bad oil cooler. Open the radiator overflow cap; if there is oil in there you have a problem. On the underside of the cap it means nothing, any engine with a long fill neck for the oil will have residue .
These aren't bad cars, but they are expensive to repair, so find a knowledgeable dealership and spend your money wisely, not brashly with some clueless shade tree.
Good luck, I love mine.
Good luck getting those pulleys covered under warranty... which by the way is no longer valid anyway.
If you do have oil in your overflow, the oil cooler is busted and needs to be replaced. It's under the intake manifold between the heads. The part at the dealership is $378. Replacing it is not really all that bad, if you're mechanically inclined.