I looking for a kind of rule of thumb here on changing the fuel filters.

Like when do you inspect the strainer? When do you change the primary, and then the secondary...

I did a search but didn't find anything other than just change them if you see a loss in fuel efficiency....Is that it?

Every spring, or about 12k-15k miles. I do them both at the same time.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Benzzy,

AS REQUIRED. Which is when a lack of power is noticed, usually when going up a hill. It feels like a stiff head wind slowing the car.

P E H

Every 20K I do both. No problems so far.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer