So here is an interesting story.
I decided that before I sold my Toyota truck (198X) I would do a quick oil change (it has a 22R).
I had been running 15W40 in it and a container of Lucas oil stab. When I drained the oil I noticed that the oil was a dark bluish colour. I've never seen blue oil come out of a car before. And it certainly wasn't blue when it went in.
Anyone have any probable causes?