[skyship007]

There are 2 reasons why used diesel engine oil might not turn very black, firstly the one I mentioned before, lack of detergents and secondly how well the oil functions in terms of cleaning the rings. If the rings are kept in good condition in both wear and Carbon or varnish deposit terms (The oil scraper rings tend to get
varnish deposits and the compression rings can get Carbon deposits if short tripped a lot without any long periods of WOT on the motorway). The condition of the injectors is also important in blowby terms, particularly if the rings are worn.

The engine oil in a petrol engine should defintely not turn black, even after 10K miles, although the oil in my Twingo 1.2 (16 valves) was real black when I bought it, although that was cos the previous owner had not changed it for 30K miles! I was surprised it had not sludged up, although it did result in a certain amount of wear to the cam phaser (Similar to a VVT) bearing. I can hear it at idle but it's not bad enough to be worth changing.
Petrol engine oil does turn a sort of dark brown in some cases, but often looks like new oil when changed.

Like the oil from a diesel it should not smell of fuel when you sniff a hot dipstick, like wot I like to do every Sunday!