Lubrication and screw threads for PLA plastic?

I am in the process of building the working 5-speed manual transmission shown at http://www.thingiverse.com/thing:713815
I need lubrication (oil) that is compatible with PLA plastic. I use PolyMaker PolyMax.
Eric didn't specify self-tapping screws instead of machine screws. I should have bought self-tapping screws. I stripped the threads in plastic. There is no room for nuts. How can I fix the stripped threads?

I have tried the metal4plastic inserts and they work great. If you have enough material, you can predrill and heat the insert and fit it in place.

EDIT : something like that :
Attachment 8936