I just changed the transmission oil filter and even though the big nut on top of the canister is tight, it seeps a bit of oil from under that big bolt.  The transmission oil is maybe 1/16" below full.  I just got this dozer and I really didn't notice if it was leaking before as all I did was drive it to where I started servicing it.  Any thoughts on what could be going on.

I removed the plug and flushed the filter housing with a bit of diesel and let it drain good before installing the new filter.

Sounds like the o-ring on the bolt is perished. I don't have a parts book for the 13A but any the 14A and 15A uses 8F-8489 which translates to 3H-0107 but any hydraulic shop should be able to match one.

a pinched or out of place o-ring will leak too. ive had that happen on canister type filters. a little o-ring lube (dow and rocol make a good one) helps, so does a light smear of oil.
make sure all the old seal is removed too. copper sealing washers should be re annealed if your reusing them best to replace em and never reuse a fibre washer they tend to crack.