I'm not telling you what to de here, it's just a cautionary tale.
When I rebuilt my 400four engine, I replaced a lot of stuff like primary chains etc. I also ordered from DSS, all the seals. Engine went back together without too much drama but there was always a mysterious oil leak, which pissed oil over my right boot. I tried for ages to find the source but couldn't.
I then had the idea of pressurising the crank case with compressed air up the breather hose. This caused oil to seep out of the gearchange shaft oil seal. Although it was a new seal, it had been on the shelf for decades and had gone hard, with the lip cracking/not sealing.
I replaced it with a fresh rubbery seal from a local seal dealer (Sealmasters in Cardiff). Job done
Here endeth the lesson