Because it is a Conversion and the Conversion does not use Oil Filters made for that specific Engine; and the OP made the suggestion that the PH8A Filter would interchange with the ones He was using I do not see how the OP could win His Claim. ("They didn't carry these filters, so I mentioned that the fram PH8A filters were compatible.")

The OP would have to find that they did something else wrong or that there was something wrong with the Filters they used that made them not work like the PH8As would have.

I think the OP would have better luck just stating the Facts on a sign on top of his Vheicle and park the Vheicle near where it can be seen by potential customers. As long as the Sign sticks to the facts like; I brought my Car here, they changed the Filters and after that my Engine was damaged due to loss of Oil Pressure; they would have trouble if they sued you for deformation.

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84 300D, 82 Volvo 244Gl Diesel