Hi All, As I have an old diesel and do pretty much exclusively long journeys (50 miles +) and live in a warmer climate I was experimenting over summer with up to 40% sunflower oil mixed with diesel, particularly when out of Spain where the price difference is 60%. In general mpg was not affected and the car smoked less and ran smoother, something that apparently is normal so all good Now it's winter, and especially given only 4 of 6 glow plugs work I have stopped using the veg oil and been putting pure diesel in. However, my father also has 15L of 15yr old 15W/50 oil which is too old (I presume) to use as actual engine oil which I collected from home so I've been adding 1L in with some tank fills when I can be bothered....I'm not sure if it's a placebo affect but instead of make it run worse as I thought it might it seems to make the car idle smoother and the car quieten down a bit, and hasn't made the car harder to start/smoke at all. I've heard on here people putting 2 stroke oil in diesel and saying it makes it run better (I have no idea what 2 stroke oil is) but would normal motor oil have the same/similar affect? By the way, no need for the "I'd only use pump diesel, it's a false economy, sounds like a fuss for not much gain" type comments, i just want to know the technical side of how it works, pros and cons as I find it all interesting. My use of veg oil, oil and 200L of free diesel I got given with some water in it has already saved me the value of the car and with zero problems so far. Besides, I hate wasting things so if they can be used to get the car a few extra miles then it is something I like to do even if the cost saving is negligible.