I was quenching my 1084 knives in old motor oil for 2 years before a guy at the Ohio forge meet asked to see my quench tank
He took one look at the sludge ring around the rim and said that was my problem the whole time, so what are you all using for fast quench oil that won't break the bank?
Bro that sludge ring is the whole story right there. Old motor oil just does not cool fast enough, it is way too slow and you end up missing the hardness window on 1084 every single time. Parks 50 is the good stuff but yeah it stings the wallet. McMaster Carr has a fast quench oil that a lot of guys swear by and it costs less than Parks. Canola oil is honestly not bad for 1084 if you heat it to about 130 degrees first, it is cheap and you can swap it out when it gets gross. Whatever you pick, keep it clean and stop dumping used oil in there.