It will burn okay, but it lacks lubricity for the injection system. How much and how long depends on which system, but as the manufacturers fine tune the systems, raise injection pressures and reduce clearances to meet new environmental standards, its becoming less and less. You used to be able to mix oil with kerosine and you probable still could, IN AN EMERGENCY ONLY. I wouldn't do it as a matter of course, but I think it would be preferable to running out of fuel and running the system dry. I'd hazard a guess at about a quart of 30 weight oil to 10 gallons (~50:1).
