I've got a Simpson 3600 psi pressure washer with a AAA triplex pump. I need to change the oil in the pump and Simpson recommends their pump crankcase oil which is ridiculously expensive. The manual states that if the Simpson oil is not available, use SAE 15W40. Should I be using NON-detergent oil in this pump crankcase...or should I just get some Rotella or Supertech 15W40 for this application. Thanks!!