Upstream oxygen sensor part numbers

2000 XJ, 4.0, AW4, NP242, Federal emissions.

I'm having a bit of a problem figuring out which upstream oxygen sensor to order for my XJ. The ones that are on there now are old enough that I can't read the stampings on them, and the Bosch partfinder is listing two different suitable numbers: 13100 and 13399.

Could someone please explain to me what the difference between the two is? Unfortunately I'm going to have to mail-order these (local prices are double what I'm finding online), so would prefer to get the right one the first time around. Fortunately there's only one choice (13138) for the downstream one, so at least that's easy.

13399 is supposed to be an Original Equipment type.

13100 is supposed to be and Exact Fit replacement.

There may be minor differences in the wire plugs or you may need to splice in the 13399. I would guess that the 13100 plugs right in.

I just replaced both my o2 sensors, and the 13100 is the one I ordered for my '99.