Oily residue behind fuel caps

[DesertNomad]

My Dakota has locking fuel caps and the O-rings used to be white. Because they were old, hard and would allow fuel to seep out if filled too full, I replaced them at annual. My A&P replaced them with black ones.

Now occasionally after flying, fueling and parking overnight, I am getting a brown oily residue emanating from the fuel caps. I think it is the rubber breaking down. When I remove the caps and look at the seal, it is wet. A similar report (though with the white ones) is here:

https://www.aircraftspruce.com/catalog/appages/gascapgask1.php

"Not as good as the Piper originals (462-056 ) - left oily like residue behind each gas gap after flight, source of residue unknown but once replaced with piper # 462-056 issue was resolved. Guess you get what you pay for - $3.85 ea versus $9.25 ea"

The invoice from my shop shows only part number CA66815-000.

Piper seems to make two gas caps (locking and non locking) and maybe they use different gaskets? Mine are the locking caps.

Gasket: 66815-00
Gasket: 39829-02

Thoughts? They seem to fit correctly and the caps feel snug when closed.

 

[Tweety]

 

[DesertNomad]

 

[Mark Ballentine]

 

[DesertNomad]

 

[Mark Ballentine]

 

[DesertNomad]

 

[DesertNomad]

 

[Mark Ballentine]

 

[Tweety]