I've had to tear down my Canon A490 pretty much to individual pieces, in order to get a whole load of sand out of the works. In the process, I've probably washed off most of the grease that was in the zoom drive gearbox. (I didn't put it there; I bought the camera new and now when I opened it up there was grease - and sand.)
My instinct would be to leave plastic gears unlubricated, but the first stage is a worm on the motor's shaft driving a helix gear, and I guess that makes for a lot more sliding friction than in the subsequent stages - all spur-to-spur. Indeed, most of the grease seems to have been (and some still is) on the worm, but it's inevitably worked its way through the transmission as the gears have meshed.
I don't think any of the gears are oriented such that their turning could fling blobs of grease onto the lenses, but some clever IRC people have suggested I should look for a lubricant that won't condense on the optics. I also know that not all lubricants are compatible with plastic - I guess they could corrode / dissolve certain materials?
So, clever people of Photography, what lubricant should I put on that worm? There's obviously a cost involved in adding grease - it captures any grit that does make it in there. But Canon obviously decided that the worm should be greased, so I want to just go along with that.