Hmmm, I hope I don't step on too many toes here, but I would like to give a little input on a few bits of info here. What I am putting forth will only be what is recommended by the manufacturers, industry and what I have confirmed with years of experience and testing in my own shop. I have seen the 1084 labeled deep hardening before, and this is a misconception that needs to be cleared up (sorry Jeremy, no disrespect intended). The concepts and terms of "deep" or "shallow" hardening is based upon the Jominy end quench method of describing the hardenability of a given steel. 1084 is a 10XX Series steel and, along with all other 10XX steels is very much a shallow hardener. Any oil not in the 7 to 10 second range, like Parks AAA (11-14 second) will leave plenty of pearlite colonies in any piece of 1084 that is much over .187" in cross section, or even less. I have seen 1084 referred to as deep hardening several times before, and I am not sure where this idea originated considering the chemistry and behavior of the steel. Park Metallurgical/Heatbath #50 is matched with any steel with a 10 as the first two numbers (simple carbon steels) and the W (or water hardening) series.
Parks #50 was developed as a much better alternative to the treacherous nature of water for steels that were initially designated as "water hardening", or ones that would be shallow hardened in the Jominy test which actually used a water jet/spray as its coolant. Alloy steels that easily hardened to full depth were deemed deeper hardening and were designated "oil hardening".
Which brings me to the next point, (sorry John, but you know we are cool, and I know you get it) that #50 is definitely designed to replace water, and not oil, in hardening steel. Using Park#50 on deep hardening steel is a serious mismatch despite how lucky many folks have gotten. I too have used Parks #50 on deeper hardening alloys and managed to survive the ordeal. But this is like the folks who say they have smoked their entire lives with no ill effects- all this means is that they have been lucky, it does not change the fact that smoking is an incredibly hazardous habit that has killed thousands.
I am only pointing this out because I have seen a trend in our business to use Parks #50 as a panacea to avoid buying another quenchant when working with a steel that is ill suited for water type quenching, this is not the best approach and there is a reason that different speed quenchants were developed for different steels. Using the same quenchant for a vast array of alloys, from very shallow to very deep hardening, is like keeping your drill press on the same speed regardless of what size bit you are using, or what material you are drilling. You can do it, but expect to buy a whole lot of drill bits in your career. I would say that if any makers are set on using Parks #50 on any steel from 1095 to O-1 that it would be cheaper just to get a gallon of canola and call it good. After all, one doesn't need the expense of a Bridgeport mill if they will never change the speed of cut.
Luciusx5, in my experience, Parks #50 is ideal for both of your steels, i.e. 1075 and 1084, I spent a good six months last year looking at 1084 under a microscope and #50 yielded optimum results. If you want to work with hamons, 1075 in Parks #50 will allow you to be a Rembrandt in selective hardening. With either of these steels, if you find the hamon working under the clay, keep that oil and just adjust your pre-hardening thermal treatments and hardening temp to affect the responsiveness to the clay.