I was poking under my truck trying to figure out which heat shield was causing a buzz and just happened to notice that there were three bolts missing out of the oil pan. IIRC, they are towards the front end along the passenger's side and between two of the missing bolts is a bolt that doesn't look like the rest. All the bolts that are there have a tall shoulder in addition to the head but this one that is in the middle of the empty holes seems to be just a plan hex head. It looked like the factory bolts might have some kind of o-ring between them and the pan but couldn't tell for sure. My question is should I try to find the factory style bolts or just get whatever works from the hardware store and does anybody know what the dimensions are for those bolts?

 

I would expect to see a bolt with an integral washer. A washer-head bolt. If you haven't done it already, I think you will need to pull another bolt out and measure it, or match it up. If you can't find a washer-head bolt with the right thread spec at a parts store, then at least use a flat washer under a regular bolt to more evenly spread out the forces.
An advantage of a washer-head bolt, besides spreading the clamping force out, is that they have fine radial ridges under the washer's head, which tends to minimize them loosening up. A flat washer by comparison is, well, only flat!

 

The factory manual had the thread spec (1/4-20 for the smaller ones) and I found some bolts at ACE that had and big shoulder and a black oxide coating that will do the job well enough.