@Switch BlaydeI hit this in 1980, so have the opposite experience:
"Yes, Ma'am. Some friends asked me to help. Oh, and a tube of KY Jelly." KY had just become available over-the-counter, I was pretty sure. There were boxes of it stocked with the condoms, anyway. Unlike my later years, when there were plenty of choices for lubricants, it was the best available in 1980. You had to brave the same scowling gazes to get it, though. Might as well get some now. We could stash it somewhere in case anyone needed some.
I had a lot of fun with my characters' first trip to the drug store to buy condoms and K-Y Jelly - and so did they. Of course, it's a do-over, so you can get away with split perspectives.
By and large, it depends on the subject whether I'll toss in an explanation. K-Y got one, for instance, but other things haven't.
I have gotten some interesting questions from readers. The most common ones have involved phrases and whether or not they'd have been in common usage at the time. Those are often harder to explain.
Obviously you can't get away with a direct commentary outside of there being some time-travel aspect, but I agree with others that, mostly, you should set things where they're set and let the chips fall where they may. Anything set significantly in the past will have things that may confuse people.
I'm not in favor of footnoting, in general. Let readers ask, or Google up the answer for themselves. I tend to find footnotes pull me out of the story much more than surprises like that.