The military used engine oil in the hydraulic system because logistics was more important to them than lifespan.
Therefore many switch to hydraulic oil. It is generally not advised to mix oils as additive incompatibilities occur, so you should plan on a drain and refill. Personally I am going to replace some bad hoses when I convert so my drain is even more complete.
I given how expensive some parts are for the FLU, I don’t understand using the cheapest oil possible. Hydraulic oil will likely be a one time expense in your ownership. Use something quality.
Here is an interesting video on what is in the cheap oil