I use one of those cassette tape adapters to play my iPod in my car. (You know, put this tape in the cassette deck, which has a wire with a minijack at the end, plug the minijack into whatever you want.)
The problem is that the thing makes this infuriating squeaking noise as the cassette is being played. There must be some way to lubricate the cassette to get rid of the squeak, but I haven’t been able to find anything via Google on what I should use.
If I give the wheels in the cassette adapter a shot of say, WD-40, am I going to harm anything? If not WD-40, what should I use?
I wouldn’t do that, particularly if you play actual cassettes in the deck from time to time. WD-40, being of very low viscosity, tends to get spread around and is likely to get on the head proper. This, in turn, is likely to cause tape jams. Not good. If you can get to the parts which are squeaking, I’d try a dry graphite lubricant instead.