Ok .. so assuming this is relatively normal, what is considered the most damaging to a diesel engine and should oil be changed more frequently than a petrol equivalent. i only do 5000 per year now.

You could change it every day and it would still be black so change as manufacters recommend you will not go far wrong!

Changed my oil and filter at 138,000 miles 6k after last change. Warmed up engine nicely and left plug out for hour as suggested whilst washing whole family's cars. Poured in exactly 5.5 litres and ran engine for 10mins. The oil was so clean I had trouble reading level on dipstick in wintry sunlight. I'll be happy when it goes black so I can see it's there!

Just changed my oil and I though back to this recent post.
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There is a design fault with the sump and sump plug. (from the Peugeot / Citroen base engine designers).
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The sump drain plug sits in an upward recess so that it flat face is flush with the sump base. This in turn is exacerbated by the thread itself then being higher up still. With the car horizontal and the drain plug removed you are always going to have a large mug full of dirty oil in the sump that will never drain out hence the new oil gets blackened by whats been left in.
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so......
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The trick is to jack the car up at the right hand front wheel, tipping the car over and up so that the sump drains more oil out. THEN I always get a furry 'pipe cleaner', fold it in half, then fold 30mm back like a hook. Tuck this up the drain hole such that the hook rests inside the bottom of the sump, go for a cup of tea whilst capillary action sucks the remainder oil out, you will be amazed at how much more oil comes out extra to what you thought was empty.
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Last trick; if you forgot to buy a new copper washer, use a blow torch and heat to cherry red (just the copper washer), and dunk immediate, (whilst glowing cherry red), in cold water, polish clean with the abrasive of a kitchen sponge. This heat and quench action softens the copper and it will give a better seal when you tighten it up. it will age harden again quite quickly.
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Hope this helps.
Kind regards from Fred.
. 2010GS Auto ex Celeb, Santorini Black, 2011 facelift , spoiler, spats, mudflaps, LED footwell lamps, Witter detach,
Other cars:-
1958 & 1959 Austin J40 Pedal Cars under restoration (I make many J40 parts)
If you have an Austin J40 or Pathfinder Pedal Car looking for a new home pls PM me.
2009 Meriva

Good tips.