To be honest, I'd be wary of using anything not recommended by the manufacturer. Remember, they perform extensive testing on their products before releasing them to the market, and they make recommendations for good reason. Beside lubrication, one of the main mechanical properties of a oil is its ability to resist shear stress. Another is to remove heat from component parts, i.e. the underside of pistons, etc. If an oil is too thin, its ability to resist shear stress is reduced, it will break down, form a film, burn and accelerate wear. Think of the pressure the big-end and con-rod bearings are under every time the fuel ignites in the cylinders. At 1500 rpm, those bearings are resisting that shear stress at 25 times per second... a lot of work. I haven't got the energy right now to write a full explanation, but try this article, and see what they say about thin oils and diesel engines. Sorry, tried that link and it didn't work, so try Googling: www.jeepfan.com/tech/motoroil - faq.htm My link Good luck.