What wieght oil do you use?

BTW Adam most diesel spec oil is 15W-40. It's ok for a gas engine but there is better. Diesel oil has additives to handle the soot that gasoline oil doesn't. Also gasoline oil needs some stuff to handle the unburned hydrocarbons that diesel oil doesn't.

Just like air cooled motorcycle oil and aviation oil, they all have their place and there's a reason they make all those different kinds of oil. Oil, in this case is not just oil.