I would argue that in most older traditionally built vehicles it just doesn't really matter. In a hydroboost system maybe it does because of the added strain on the fluid. I do run a universal type P/S fluid in my truck but I've known guys that swore by DEXIII andd others DEX6 but we all have one thing it common. Our P/S systems are working just fine. Just think about Ford for a second. Especially the 90's era Explorer and Rangers, didn't matter what fluid was in them all you had to do was look at them and they'd foam. Or go up a hill and they'd push P/S fluid onto the exhaust manifold. I sure am glad I'm a Chevy guy....