Rapeseed oil works well, but will get thicker with each use until it is like golden syrup.

Funnily enough for small parts I use a half-full golden syrup tin, as you can use the lid to stop the smoke/extinguish any flames.

Do work outside, it's a smelly business.

Used engine oil can be heavily loaded with carcinogens, you don't want to inhale that smoke.

Rapeseed oil works for oil blacking as well.

Neil

The cheapest oil would be whatever you have kicking around the workshop already. Or a large cupful of cooking oil from the kitchen – it's free.

I just went to local car spares and bought cheapest engin oil. I pour it into an old deep fat fryer tank and when finnished pour it back there is a spout on fryer tank to make it easy. You need quite a lot of oil to quench the gauge plate quick enough and move it around vigorously when plunged into oil.

David

Fuel in engine oil is a real problem. A particular case is modern diesels with particulate filters (DPF) some designs (Fiat, GM) inject diesel on the exhaust stroke to regenerate the DPF and some washes down the bore into the fuel. This can give a noticable increase in oil level.
A cheap oil with minmal additives would be good. The oil volume compared to metal mass will affect the temper. So will the oil temperature. Pre-heating the oil with hot scrap is sometimes done for critial parts.

The oil quench at tech held a few gallons of oil, a friend discovered that a 20mm red hot bar could be plunged in to vaporise the oil and the quickly pulled out while still hot enough to ignite the vapour cloud, this was highly entertaining until the instructor appeared round the corner

Mike

I use Quenchall 22 oil which is formulated for the job.

I bought 5 litres many moons ago and it is still going strong.

According the the blurb it is very much like a hydraulic oil in terms of viscosity and high temperature capability.

It never flashes off and doesn't get thicker over time.

Was not as expensive as motor oil if I recall.

Mick