One thing to consider in the future if you are going to work on MTB shifters, the WD40 route is a common approach. If it fails, an alternative is to just replace the shifter. On six and seven speed bikes, I use the Shimano Tourney trigger shifters. I really like them, and you can buy them complete with cables and housings for less than $13 at Niagara Cycle. If you do much service, you will probably end up doing business with Niagara, as they are the cheapest source I have found for chains, bearings, cables and housings, and tires.

Just be prepared for SLOW service. Niagara orders take me 2 to 2 1/2 weeks to get. So I try to order BEFORE I am out of parts. I screwed up recently, and am now out of chains (did a lot of bikes recently).

I wish I would have picked up bike maintenance as a hobby back in my college days. Instead of working minimum wage, I could have made decent money, on campus, on my schedule, working on bikes.