This is bizarre. I need to remove the filler plug on the rear axle of my 91 F150. It takes a square drive key that is smaller than 3/8 and bigger than 1/4. My local parts store never hear of such a tool. What’s the story? Is it metric?
All I’ve ever used is a 3/8 drive extension or rachet.
Try cleaning out the square drive hole, it’s probably full of rust.
You might have to jam it in there a bit, as I had to do the first time on my 94 full-size Blazer. Turns out it was full of crud since it hadn’t been removed in a LONG time.
You could try using a small flathead screwdriver to clean out whatever might be stuck in the hole.
It is most likely 5/16. I ground down a cheap 3/8 extension the first time I ran into this problem. I since bought a 5/16 from Snap-On.