Just in case someone finds this post and is interested in using an oil pressure switch to control the feel of power to electric chokes on a 6v or 12v system, here is the final solution:
NAPA: MPE OP6626SB
I have this now working to control two Carter-Weber Motorcraft 740 2-barrel carbs on a 6v 261 engine.
I had to try several two-pole and three-pole switches before I got one that switched appropriately for this application. It switches on at about 4 psi (rated to switch on at 2-8 psi).
Tim