30 minutes of cranking? good way to burn a starter even if its not constant.
Try replacing the glow plugs and make sure it cycles for 8 seconds or so.
Either is a no no and you will ruin the engine because the glow plugs will ignite the either prematurely and you wil blow something in the heads.
Propane might work better for starting and while you are at it you might as well just hook it up full time. More power is better right? If money is tight then a camping cylinder will work. Dont blame me if something blows!