Yep, WD40 is the bane of anyone working on an old pinball machine. I've seen it where it looked like a brown glue over everything because it had dried up.

The manufacturers used to sell a special lubricant for stepper units, but the closest you can get now is a product sold by Steve at Pinball resource.

I'll hit it with brake cleaner to knock off the dried up crud, then hit it again with a good lubricant.

Everyone has their personal favorite stepper lube, but I like to use electronic tuner cleaner/lube (comes in a small spray can), or any clear teflon-based lubricant.