Dog??

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Very likely. And it's not like the oil filter is expensive or difficult to change, you can then draw your own conclusions about any other maintenance....

It could just be an imported filter..................

[Jaws]

Another point, on a different Bongo, I saw the oil filter was covered in Japanese writing. I am assuming the most likely cause of this is that the oil filter has not been changed since import??

Not necessarily: the last new filter I put on was made in Japan
Probably better off checking the oil on the dipstick black+smelly+dirty filter=likely lack of TLC; not infallible I know, but if I were selling I would make sure I had changed oil and filter shortly before