Those thin washers are hard to come by (mostly a dealer item), and easy to bend/destroy. The prior owner/tech probably bent or otherwise trashed the originals and didn't have any to replace with, so just used some rthread compound or something. There's a small oil passage very close to the top of the threads, so be careful if you use anything (e.g., to stop tackle an oil leak) that you do not block that oil port.
I had a leak on one of my tensioners after replacing (I think it was the rear one). Following advice I found on the forum (maybe a post by StreetRod2000?), I ordered a rubberized washer for that tensioner. It eventually worked, but wasn't really suited to the torque required. Use the thin washers (and get an extra one or two if you can from dealer) if at all possible.