I actually wouldn't be too worried about the alcohol damaging things... I'd be more worried that the gunk is designed to help protect things, and removing it would remove that one more thing protecting the board...
Then again, anything like that that's 20 or 30 years old might actually be one of those things that is actually causing a problem rather then preventing it at this point....
It's your call, but unless you think it's going to be a problem I'd leave it for 2 reasons:
1. I'm lazy
2. There must be some reason it was done that way.
If you do decide to give it a bath, alcohol and plastics have issues, so you probably don't want to submerge the board to the point of submerging the chips, but even if you do it'll probably be fine as long as you make 1,000 percent sure it's dried well before you attempt to use it again. I'm also unsure when the board will start absorbing the liquid (I was going to say if, but google pointed me to when), and weather that will be a problem...
In summary, if you really want to clean it, you're probably better off scrubbing it rather then letting it soak, though I'd probably leave it. But if you do soak it, it'll probably be ok, as long as you don't let it soak too long...
Chris