got the truck out for a test drive today and the fan seized up on it. I got it to turn again but what kind of lube should I put on it or in it?
What engine/fan do you have? Most early flatheads were either crank or generator mounted. Isn't anything related to the fan that can lock up. Generator bearings can go bad, but that's about it. If its a late series 8BA/8RT/8CM: on the hub of the fan you'll find a round head machine screw. Turn the hub so the screw is at the top. Remove the screw, add an ounce or two of motor oil and turn the fan so the screw hole is down. Catch whatever excess oil drains off and replace the screw. If this is a late fan that locked up you may be looking at replacing bearings/seals.
A buddy of mine had 2 like yours seize up on him. He finally machined it for a needle bearing and had no more problem. I was planning on adapting mine the same but found that it already has a bearing in it. It's a 49 Merc. Henry had a lot of different fan configurations. Good luck,