Making a new part might be the way to go, the stock one don’t look that well made. As far as hardening off parts I’ve only ever used , used motor oil... from a diesel truck/ transport, something with a lot of carbon in the oil... maybe too much/hard..?
Do some reading on heat treating. You won't find anyone but rank hobby dudes, and likely they learned from someone that knew less than they let on, that would recomend using dirty crappy engine oil.
You need to know what your steel alloy is first and foremost. There are air, water, and oil hardening steels, as well as precipitation hardening (they get baked, which makes them grow crystal structure which makes them harder) steels. There are steels that will not harden without infusing them with carbon on the surface, which generally requires prolonged high temperatures in direct contact with the carbon source. It won't pick up the carbon needed whilst being quenched in dirty oil.
Using clean quench fluid is a reduction in the amount of random crap to clean off the part, as well as a straight reduction in the amount of potentially toxic crap to breathe or have otherwise settling around the shop. Y'know, in the event at you were not wearing breathing apparatus, etc. Some dudes seem to just enjoy scrubbing crap off their projects, though, too.
Most places that deal in tools can sell you known steels for cheap enough that dicking about with random stuff isn't really worth it. Look at the KBC tools website, ferinstance. Flat or round stock, in a bunch of alloys, each with different heat treatment needs, to reach a predictable end result.
In some cases, the damage from quenching the part in the wrong fluid is pretty benign, like it doesn't get fully hardened, in other cases, the part can fall apart from the stresses. Which makes for a bunch of wasted time.
If you know what you are starting with, it will save you a bunch of extra work and rework.
The instructions of making something hot, then plonking it into oil or water, are really a very basic skim over what is a science in its own right. For some things, that works fine, but the failure rate can be pretty high too. Best to skew the odds in your favor as much as possible.