My car is far from overheating, but it was running slightly warm all day today.

I went for a two-hour drive in the "heat" today (about 72°), and while cruising at highway speeds, my car consistently ran at about 210-215°. In seasonable weather, it runs spot on at 177-183° (180° T-stat).

Coolant is about 6,000 miles old, and the only abnormal check engine light I have is my usual warm-weather P1870.

I'm not really worried, but if this ends up being a hot summer with plenty of 90-100° days, I don't want to find out that this is something I should have noticed.

Could be a few things.
Low coolant
T-stat sticking closed
Radiator clogged(inside and outside)

There'* plenty of coolant in the bottle, but I'll check the radiator level in the morning.

agree with mike. most of the time out of those its crap building up in the condenser and rad fins. so i take out the fans and back flush them with simple green and hose

Stock thermostat for your car is probably a 195, not a 180. In the winter time, with lots of cold air running past the radiator, the 180 tstat will keep your engine at 180. In summer, once your engine reaches operating temp the tstat will stay open, and the engine will run at its normal steady state temp of around 195 to 200.

Having said that, in case it is something other than what I describe, and just because its a good thing to do, follow Mike'* and Justin'* suggestion of cleaning the radiator fins.

Roy, I do the same thing with my t-stat. In the summer, I will run a 180, but swap it back out in the winter time for a 195. Speaking of which, I need to do that now.

I sprayed as much Mean Green as I could (check your local dollar stores!) onto the condenser and the radiator, then went to the DIY car wash and turned the high-pressure hose on them. Got a lot of the bugs out, but it didn't do all that much to help. When the hottest part of the day rolled around (a beautiful 75°), the running temperature still went up.

Now that I've had time to think about it, I think I found my own problem. When I replaced my tensioner, I drained the entire system. When refilling, I poured in a jug of straight green coolant, but was only able to add less than half a jug of water before it was full. (I topped off with water as the air worked out, but that wasn't much more.)

My coolant tester says the fluid is safe to over 280°, but would a lack of water in the system be why it'* a little warmer than usual?

it could water is what does the cooling, coolant keeps it from freezing. if you going to guess lean toward more water than coolant, also cleaning the rad from the front doesnt do much because thats just pushing the bugs, pollen, and stuff further into the rad. and high pressure hose should never be used it could very easily bend the fins then you will have diminished capacity. try shining a shop light from the back you should see light through all areas