I recently purchased a 1984 Simplicity 7790 diesel garden tractor. It has a Lombardini 18HP engine. The owners manual calls for 40W oil of CD service class (DS series 3) . I have had no luck finding ANY straight 40W oil. The only straight weight oil I can find is 30W. Can Multi viscosity oils be used in these old engines. I emailed Lombardini and haven't gotten an answer. Can anyone shed some light on this for me. I sure don't want to use anything that would damage the engine. Thanks....Nick