What year and engine?

If by any chance you are referring to this 53-63 235/261 tool (Pic 1 & Pic 2), they used to be readily available at NAPA, etc. You can make a tool by cutting a notch in the center of a screwdriver blade.

The same tool was probably also used on some GMC straight-sixes into the early 60s?

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.