Use a transparent pre filter so that you can see into the filter .I now have over 55,000 on my pre fuel filter and I see very little junk in the filter.But, the main fuel filter doesn't last 25,000. However, I use quality (Shell) fuel only unless I'm on a trip. The price of fuel is about the same locally.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback