Thanks to Mike Haughton and the Model Engineer’s Workshop magazine I’ve got an answer to a question that’s been bugging me for a few months.
“What’s the difference between thick oil and grease?”
Mike explains that the thicker the oil you use, the more load it can take, so heavy equipment or equipment such as a milling machine could use thick oil. Grease on the other hand is a mixture of oil and a soap. The soap “sticks” the oil in the place you need it and the lubrication is done by the oil. Grease is used for things like roller bearings those “fit and forget” applications.