DJ Scotty-esque levels of paranoia Just kidding, I wouldn't put that on anyone.

A hair isn't going to make much of a difference. The only thing it's going to affect anyway is the internal pressure from basic dyamics laws...and only a tiny bit isn't going to make that much of a difference in the internal pressure. The most important thing in the fluid change that most people overlook is to replace the $.10 aluminum crush washer to ensure perfect sealing.

Once at a Valvoline (no DIY changes for me in the winter) the morons filled my oil like a millimeter above the full line with oil that I had given them to change it with...I ended up getting the change for free and the cost of that oil he wasted refunded. I made them drain it a little and do it perfect too.