Where can I find a 5/16 square drive tool to take out the oil drain plug on a 2002 F-250 7.3 diesel? Thanks

 

I think I would find a cheap 3/8" ratchet extention in the bargain-basement tool department, then nudge it up against a grinding wheel as appropriate.

Pop

 

If you have a grinder, find a slightly larger diameter bolt and grind it square to fit.
Or any piece of rod, etc. will also work.
PICS of install.

 

I bought a piece of 5/16" square key stock at the local auto parts store for $3.00. Worked like a charm.

BTW, after installing the Oilguard system, my wife commented that the 'ol girl is quieter coming up the road to the house than before. I don't know if its the fresh oil or what, but I thought I was imagining the quieter & smoother way she runs now.