1981 300D

Hi Guys,

I've been having to change the pre-filter very often and it gets clogged to the point it stops the engine.

I put another filter just before the pre-filter 2 days ago and it's clogged already.

I don't know what to do. I went to a boat store looking for some additive...and they want 26 bucks for a liter...and I'm supposed to put 1 oz. for every 16 gallons. That's an expensive option.

How about the diesel doctor offered by performance products? It is something I have to use every time I fill up the tank, or is it something to be used at certain mileage or time interval?

Thanks for any input...