No. Turns out it's the bell housing gasket.

 

You’re sure it’s not a rear main seal leaking?

 

😆 Ya, it makes it stiff. No, unfortunately I've got a gasket leaking between the trannt and bell housing.

 

Check to make sure there are no gouges or starches on the mating surface. That will cause it to leak. Also, in my experience, the Allison filters like to be out on a little tighter than say a oil filter. I usually get them fairly tight by hand. Beyond what is marked on the filter. Obviously don’t crank it down and fully compress the seal, but try going a little more with it and the standard.

 

Check to make sure there are no gouges or starches on the mating surface. That will cause it to leak. Also, in my experience, the Allison filters like to be out on a little tighter than say a oil filter. I usually get them fairly tight by hand. Beyond what is marked on the filter. Obviously don’t crank it down and fully compress the seal, but try going a little more with it and the standard.

I hate it when starch gets on the mating surface

 

I thought I had the same problem, but it was actually the gasket of the external oil cooler block off plate that was leaking and running down the side of the filter. Look almost directly above your filter on the side of the block

 

Turns out I've got a gasket leak between the bell housing and tranny.

 

I didn't check the center fitting or the mating surface. The old gasket came off clean and the drip was coming down from the side of the filter. I'm going to take it back off tomorrow and check the fitting and make sure the mating surfaces are clean. Thanks

 

Did you do anything besides remove the old filter and install the new one, like check if the center fitting was still tight and undamaged, that the mating surface was nice and clean and didn't have an old gasket stuck to it, or even that the trans fluid wasn't coming from someplace nearby?

 

Turns out I've got a gasket leak between the bell housing and tranny.