Strange behavior, probably from the alternator.
Some weeks ago, some clumsy idiot, which is to say yours truly, didn't put the oil filler cap back on the right way, and at some later time it popped off and fell away, presumably landing in the middle of the road. I've replaced it now, but now I've noticed troubling signs from the alternator, which recharges the battery fine when I drive, and when testing it in my garage at idle (via a multimeter and the battery terminals), it does produce adequate amperage (yielding 14.1 volts in the battery). But if I add an electrical load, the amperage drops significantly. With the lights on the output voltage drops to 13.40, with the AC on it drops to 13.10, and with both together the battery voltage reads 12.43.
Before and after a road test with no accessories or lights running, the battery voltage, with the engine shut off, read 12.45 and 12.70 respectively.
I should mention that on the occasion when I discovered that the oil cap was gone, that wasn't the first thing I noticed on opening the hood. What I noticed first was a considerable amount of oil splashed around the engine bay, although the dipstick didn't read low. I've cleaned that up as much as I could, but is it possible that some oil got into the alternator? Would that cause it to malfunction, and is there a remedy? Will any oil present eventually drip out and everything return to normal? If not, is there a way I can clean out the alternator, or at least make it less oily?
It wouldn't totally surprise me if it turns out I need a new alternator, considering the car's age, though the odometer reading is well below average.
ETA: It's a 2000 Nissan Altima that has about 119K miles on it. I should have mentioned that. (Thanks u/HannsGruber.)