I have had success spraying a spray lubricant into the interior of the shifter and then working the levers.
Don’t use WD40 which will surely gum up in a matter of time.
__________________
1973 Schwinn Super Sport
1986 Schwinn Peloton
1976 Raleigh Super Course Mk II(for wife)
1983 Gitane Super Corsa
1991 Trek 750 Multitrack