You need a plain mineral engine oil that has no additives or friction modifiers. I can't stress enough about making sure it has no friction reducing additives. If it does, your steering brakes will simply not work, as they rely on a high friction co-efficient to operate correctly. Most certainly do not use any hypoid type gearbox or differential oils, as not only will the brakes not work, but the oil attacks the bronze bushes in the diff and steering mechanism. You could also contact Landis Zimmerman to get the good poop on local brands and types.