EDIT:
The problem seems to have been nailed down to my headgasket going bad (150,000 miles isn't bad), so I'm changing this thread to a documentation thread of me replacing my headgasket and getting some rebuild work done on my heads while they're off.
Well, my 4Runner has been acting a little pissed off lately. I'm glad I got an FSM because I'm fearing a top-end rebuild (or possibly full-engine rebuild) may be in my not-too-distant future Overall the engine's felt a bit sluggish lately, and I'm pretty sure I've noticed some power loss over the past 2 months or so. I'm about to turn over 150,000 miles, and over 100,000 of those have been supercharged so it might be due.
So not too long after I changed to Denso Copper spark plugs (I don't think the plugs are the cause) my 4Runner has been having some trouble with starting. After starting, for the first minute or two it idles VERY roughly, and it feels like one of the cylinders may be having problems with ignition. I can get rid of the problem if I rev the engine up to about 1500rpm for about 20-30 seconds, after which the idle smooths out like all of the cylinders are firing OK but the idle isn't exactly silky-smooth. The plugs are essentially brand-new (I need to pull them to help diagnose the problem) and the plug wires are a little more than a year old with something like 20,000 miles on them.
Additionally, I think I may be burning a slight bit of oil at startup, especailly on the cold mornings that are below 40 degrees or so. It's hard to tell exactly because when you start up the engine on a cold morning you get a lot of cloudy condensation out of the exhaust, but I'm pretty sure I see a little bit of white bluish tinted smoke when I start driving that hangs in the air longer than the white condensation. The smoke and condensation goes away after 1-2 minutes of easy driving once the engine is warmed up. When I got back from Breckenridge last weekend I checked my oil to find it was about 3/4 of a quart low, which goes along with a burning oil theory. On a semi-related note, my coolant is also very slowly going somewhere, I had to put in almost a gallon when I checked the oil but I hadn't checked my coolant in a while.
I have no mixing of fluids between my engine oil, coolant, or transmission fluid as far as I can tell, but I'm planning on flushing my radiator and changing my engine oil and tranny fluid for a more thourough inspection. I'm also going to buy myself a compression tester, if anyone has any suggestions on ones that work well.