Ok so I need to change out to 5w40. I have had some trouble starting the old 300SD with the temps in the single digits. I also need to let the glow plugs run there full 35sec. I have found that the block heater is kaput and there is no way it is getting changed anytime soon. What do You guys think of a lower rad hose heater? I have to change the thermostat and flush the system anyway. The valves were adjusted 6k ago and it has always started good down to 12-15 deg. The battery was new in july. I have good cranking speed for a minute or two in 15 to 20 sec. cranks.