this is a question probably best answered by our oil experts. The mechanic at our bus garage says they are replacing the hydraulic oil used in the school buses with the same 15w40 used in their diesel engines. Have been doing for years with excellent success on units that use the hydraulic oil for rad fans and power steering units. Suggests I should do the same in my Barth and only have to carry spare 15w40 for both engine and hydraulics. Any thoughts on this?

Seal compatibility would be my concern. Without specific information on every seal, it would be a risk.

Maybe hose compatibility, too. Thumbing through the Gates catalog can be quite illuminating.

There's a reason manufacturers recommend a specific fluid for power steering units. They're designed to use a specific product. Use of the wrong fluid can shorten service life.

The ATF commonly used in P/S units has no affinity for water; motor oil will carry it in emulsion, although there should be minimal water instrusion (from condensation) in the P/S system.

The viscosity and viscosity index of ATF are less than 15W/40; the additive packages are also quite different.

Seals should not be a problem - the other factors could be.

sounds like my original feelings were right and to stick with recomended oil for the hydraulic system...thanks for the input.