I've never understood the rationale behind 'fluid is too expensive so I'll use the cheap stuff'... You pay thousands for a tractor and then cheap out on fluids and filters.. I don't get that at all.
Far as Coyote Machine's comment, you ever bother to do a spectroscopic analysis on your fluid? Bet not. I do with all my fluids every time I change them. Additive packages wear out in any fluid, don't matter if it 'appears' fine or not. Only way to tell is by doing an analysis. Especially hydrostats are touchy when it comes to fluid. Gear drives, not so much but then all mine are gear drive and they get changed every 2 years, sooner if they appear milky and each of mine holds 15 gallons btw. I do keep them inside when not being used as well.
I never cheap out, ever. I depend on my tractors for farming and keep in mind that not only does the fluid lubricate the gearbox or in the case of a hydro, the entire transmission but it also provides hydraulics for your loader or any implement you have on the back that requires hydraulics plus it keeps your wet brake discs lubricated.
I just don't see being cheap on fluid when the tractor costs many thousands.