I have had a trek 7100 for about nine years and over 30k miles
. The cheap bar twist shifters are original. And work well
The trick to long life of a mechanical device is cleanliness and lube. There is not a lot to wear on a shifter so clean away. The wd40 suggestion works well.not sure if a whole can is needed. After all if it runs out clean more cleaning is a waste of solvent.
After cleaning lub it. Check if shimano has a preference . I used lithium grease on rathe shifter and derail lures of the 7100. My trek 1200 I used lithium grease on the deraillus but oil in the down tube shifters and cables . I clean the gear about every 300 miles

Likely you can do it with less--by "small can" I meant *small* (the 1"
diameter thing).  But since I didn't have a good way of measuring how much
junk was left (there's obvious junk, and less obvious) I figured a
"measured dose" approach was not unreasonable.