Speed isn’t the only thing to consider and choosing a quenching medium based strictly on steel type is a bit misleading. Blade geometry or part design and the intended end use also play major roles.
It isn’t difficult to get the thin edge section of a blade to harden, just about any oil will do. Where it can get tricky is to try and get full hardness along the spine without over stressing the edge. Hardening knife blades, (especially the most important part, the edge) isn’t as much of a problem as warping/distortion, excessive micro fracturing and quench cracking.
The petroleum based fluids exhibit 3 cooling mechanisms with different heat transfer rates, vapor phase (slow), boiling phase (fast) and convection, in that order. The vegetable oils exhibit one, convection, with virtually no vapor or boiling phases. With the petroleum based fluids what happens is you get different cooling mechanisms, with different heat transfer rates, working on different areas of the blade at the same time. This compounds the stresses associated with the inherent thermal gradients of knife blades. In other words, the thin edge section gets to the fast boiling phase while the spine is still in the slow vapor phase, just the opposite of what would be ideal. This isn’t a problem with vegetable oil, you just have the inherent thermal gradients themselves to contend with. The petroleum based fluids are a bumpier more stressful ride for the steel. Vegetable oils are fast enough, with less stress to the steel. The vegetable oils are fast during the high speed initial part of the quench because there is no slow vapor jacket phase and the difference in temperature between the blade and quenching medium is at it's highest.
During the low temperature transformational part of the quench, or slow part of the quench, the speed is regulated strictly through viscosity and at this point both types of mediums exhibit the same cooling mechanism, convection. So, which ever fluid has the lowest viscosity will cool faster and which ever has the highest viscosity will cool the slowest. Some of the super thin fast accelerated petroleum based oils like Parks #50 actually have a lower viscosity than the vegetable oils, which means that they would also be more stressful during this part of the quench.
The best things about the petroleum based fluids are that you can buy them in different speeds, (not significantly so with the common vegetable oils), and they can save you money in the long run. The worst thing about the vegetable oils is that they aren’t as stable as the petroleum based fluids and need to be changed out more frequently.
This is all just theoretic nit picking and it doesn’t mean that you can’t get good results with either type of quenching medium. Depending on a number of possible variables, in some cases the accelerated petroleum based fluids may be a slightly better choice and in others the vegetable oils. However, aside from the metallurgical advantages of the vegetable oils they are also safer to use, biodegradable, and easy to acquire in small quantities at a local grocery store.