Is a tiny bubble on the Schrader valve cap normal, or do I have a slow leak?

That bubble test is pretty telling. Trapped pressure under the cap lets go in a second or two once you crack it loose, then it stops. A bubble that forms and sits there for a few minutes is refrigerant actually creeping past the core. The oily dot coming back after you wiped it points the same direction, since oil rides along with whatever is escaping. That said, slow core weeps are super common on these little valves, expecially after a system has sat through a winter. Your temperture observation actually supports it too. When the sun warms things up, the pressure climbs, and a core seal that was fine at 8am can pass a little vapor by midday. It doesn't mean anything is dying, it just means the core is doing a mediocre job.

Good news on the swap question: you do not need to recover the charge. A core removal tool threads onto the service port and has its own shutoff, so you can pull the old core and drop a new one in with barely any loss. We're talking a tiny amount, nothing like a full pump-down. The cores themselves cost next to nothing and the whole job is five minutes once you've got the tool. If you'd rather not buy the tool, a cap with a good gasket really is part of the sealing setup and will hold back a slow weeper for a long time. Plenty of systems run thier whole life that way. But since you've alredy got the oily spot reappearing and a bubble that lingered, I'd just swap it and stop wondering every time you walk past the unit. One tip for after the swap. Soap test the new core with the cap off, then snug the cap back on hand tight plus a small nip with pliers. Don't crank the plastic cap hard or you'll split it.

Been on the other side of this, so I'll add my story. I saw the same oily spot on my garage unit and talked myself out of it for two summers. Then in August it stopped cooling as hard and I finally had someone come look. Low on charge, and sure enough it was that little core weeping the whole time. The tool that ended it for me was the locking core remover with the built in valve, bought it after that scare and did the replacement myself in the driveway with the system running. One thing I didn't know untill the tech told me: make sure the replacement core is actually rated for the refrigerant in your unit, they aren't all interchangeable. Took longer to unscrew the caps than to do the actual swap.

Mine did the same thing a couple years back, little bubble that just hung there. Swapped the core and the oily ring never came back. One thing I'd check while you're at it is the inside of that plastic cap. There's a small rubber seal in there and mine was squashed flat from being cranked down too hard by the installer. Once that seal goes, the cap can't back up even a healthy core. Replacement cap was cheap and it's held fine since. Whole job took me maybe ten minutes once I actually committed to it.

Agree with the guy who waited two summers, don't be that person. I put mine off for one season and by the next spring the oil dot had turned into a little ring around the port. When I finally pulled the core out, the little pin inside had some gunk crusted on it and the seal looked roughed up. Swapped it in ten minutes and the spot never came back. The tools are cheap enough that it's not even a decision, just do it before winter so you're not thinking about it every time the unit kicks on.

Update: went ahead and swapped the core. Turns out I didn't need to recover anything, the shut-off valves on my line set let me isolate the charge, so it was a ten minute job. New core plus a fresh cap seal, since the old rubber one was flattened like that other guy described. Bubbled it again after, nothing. Oily dot's been gone a week now.