2002 saturn L300 with Oil in the coolant.
First, no coolant flows through the intake on the 3.0L in the L300 and Catera. The intake is a 5 piece setup, the base of which is a phenolic spacer to isolate heat from the intake manifold.
Beneath the phenolic spacer there is an oil cooler installed between the cylinders inside the block. There is a cover with 2 oil cooler lines that run up from the right side of the engine to the cover and into the cooler via an inlet and outlet banjo bolt. If the cooler wears out there will be an exchange of coolant and oil, more oil into the coolant than coolant into the oil, although there will be some.
You might notice a small drop in performance, and later will notice that the coolant reservoir is full of oil and is spilling over.
I'd start by checking cylinder compression to make sure you don't ALSO have a blown head gasket, but the cooler is quite notorious for blowing out. If you do decide to replace it you will need:
1: a new oil cooler
2: new gaskets for the steel cooler lines, they will be metal crush gaskets. 4 as they are for the banjo bolts (2 washers for each bolt, one bolt on each line)
3: 4 new gaskets for the coolant bridge (the almost crescent shape aluminum bridge next to the oil cooler cover, aluminum with hoses attached as well as the coolant temp sensor)
4: New gaskets for the banjo bolts on the coolant bridge, they're rubber but don't just use some junk! Get the real deal ones from the dealer and save yourself the trouble of redoing it later on. They're cheap.
5: Oil, Antifreeze and a new oil filter. I'd spring for the Coolant system cleaner if it's not a big deal, less oil in the cooling system makes for a better performing engine and better heat in the winter.
Tools: 3/8" ratchet, standard set of extensions for the ratchet, set of Torx sockets and inverted Torx sockets, set of 3/8" sockets, flat head screwdriver, dikes or side cutters.
1: take off the intake manifold clamps by prying where they latch together, don't damage them unless you plan on replacing them (your call, but a tech would reuse them)
2: take off all the Torx bolts for the manifolds, there's 4 pieces to the aluminum manifolds and the ECM and Vacuum reservoir have to come off the rear manifold to get to some of the bolts. NOTE the location of all vacuum lines and coolant lines so you don't forget them later. Also note the position of the ECM on the manifold so you don't have to figure out which way it goes on later, it only plugs in one way but it's a pain if you fumble it up.
3: Take out the phenolic spacer (black thing under all the manifolds, feels like plastic). There are rubber gaskets on these, if they're damaged then it's a good time to replace them, if not they are usually reusable. Napa, Advance Auto, Auto Zone, etc. always have them. Just ask for intake manifold gaskets, even though they're really phenolic spacer gaskets.
4: Remove the banjo bolts for the oil cooler lines, note the placement of gaskets on each side of the line that the banjo bolt goes through. You'll want to remember that later to avoid a leak. Also remove the coolant bridge banjo bolts and move it out of the way, again noting the gaskets for later replacement.
5: loosen the 2 nuts where the oil cooler lines were, these are attached to the oil cooler and secure it to the cover.
6: Remove the Torx bolts for the cooler cover, lift the cover, and remove the cooler. Make sure you clean that galley out real well, saves you some headache when cleaning later on.
7: When reinstalling the oil cooler cover remember to use a 2mm bead of sealant around there. I've seen guys use Permatex Orange, but if you can swing it then get something better from the dealership. You don't want to do this again in a year if the Permatex leaks on you.
8: Reinstall coolant bridge, oil cooler lines, phenolic spacer, manifold(s) and clamps. Double check all your work! Change your oil, and flush out the cooling system.
Estimated time? For a tech.. probably 2 hours. For a Shadetree mechanic? Probably 4 to 5 hours.
Good luck!
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