Oil question - cabsmanuk
What would cause oil in a diesel engine to thicken and become 'gel-like'? It hasn't happened to me but it has been suggested that it could. Engine is a 1.9 tdi VW unit with full service history and 2500 miles to go before next oil change.
There is a long background to this question and not realy worth repeating.

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Oil question - GGH
If it is a mineral oil, it might have been overheated.
Oil question - J.B.
Diesels produce lots of carbon which makes the oil very treacly to feel but I've never come across actual gel. Even a newish engine makes its oil very black very quickly. Water in the oil emulsifies it in places like the rocker cover.This could be described as gel I suppose.It would never stand the shear forces in the engine so it wouldnt feel like gel on the dipstick.
Oil question - Dizzy {P}
My old Triumph 2500 (ex-PI) stood in the open from 1986 until 1996 without any oil change, though the oil was fairly clean at the start. When I came to restore the car, the whole of the oil had turned to a gel and had to be 'dug out' of the engine. I believe this was due to bacterial action, as used to happen with some cutting oils that hadn't had a bactericide(?) added and hence made an awful smell.
Oil question - Cliff Pope
I had the clutch fluid in my Triumph 2000 turn to a jelly overnight.
It was probably very old, being the old crimson stuff. One day the clutch was working perfectly, next the whole system was full of cranbery jelly. 'Dig' was the right word - I had to strip both cylinders and rod-out the pipe.
I never realised it was bacterial action.
Oil question - Claude
Bacteria is the most likely cause.

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