Hi Sam

>>>>I changed the oil yet again last night. Pitch black.

What goop have you been chucking into your engine? Sounds like the goop that
normally clings to the engine walls is starting to flake off & clog up the
arteries. My engine was burning 100% with no oil leaks, but when I flushed
it I disturbed the goop which the PO had regularly added and I had to change
the oil & filter 3 times before the buzzer stopped buzzing! Each time I
couldn't believe how black the "new" oil came out.  And use 20W50/60 oil.

If that doesn't work.....
Check your wiring from both sensors. After 13 years the insulation around
that wire is rock-hard and brittle and may be shorting against the block.
Sometimes reversing causes things to vibrate differently! It's easy just to
slide a silicon sleeve over the wires if it's just insulation cracking up.

Good luck
Regards
Nigel