What does my oil actually do? The point of engine oil is primarily to stop all the metal surfaces in your engine from grinding together and tearing themselves apart from friction whilst transferring heat away from the combustion cycle. Engine oil must also be able to hold all the nasty by-products of combustion, such as silica (silicon oxide) and acids in suspension. It cleans the engine of these chemicals and build-ups, and keeps the moving parts coated in oil. Finally, engine oil minimises the exposure to oxygen and thus oxidation at higher temperatures. It does all of these things under tremendous heat and pressure. How do I read the numbers around the 'W'? For example 5W40? Single grade oils get too thin when hot for most modern engines which is where multigrade oil comes in. The idea is simple - use science and physics to prevent the base oil from getting as thin as it would normally do when it gets hot. There's more detail on this later in the page under both viscosity, and SAE ratings. But as a quick primer - the number before the 'W' is the 'cold' viscosity rating of the oil, and the number after the 'W' is the 'hot' viscosity rating. So a 5W40 oil is one which behaves like a 5-rated single grade oil when cold, but doesn't thin any more than a 40-rated single grade oil when hot. The lower the 'winter' number (hence the 'W'), the easier the engine will turn over when starting in cold climates. A quick guide to the different grades of oil. What the heck was Black Death? Black Death first appeared in the early 80's when a horrible sticky black substance was found to be the cause of many engine seizures in Europe. It was extremely frustrating for vehicle owners with this problem. Dealers and mechanics had no idea what was going on and Black Death just wasn't covered under insurance - if your engine had it, you paid to fix it yourself. Many engines were affected but Ford and Vauxhall (GM) suffered the most. Faster roads, higher under-hood temperatures, tighter engineering tolerances and overworked engine oils turned out to be contributors to the problem. The oils just couldn't handle it and changed their chemical makeup under pressure into a sort of tar-like glue. This blocked all the oil channels in the engines, starved them of lubrication and caused them to seize. I don't recommend this but you can reproduce the effect with a frying pan, cooking oil and a blowtorch. The cooking oil will heat up far quicker than it's designed to and will turn to a sticky black tar in your pan. Either that or it will set fire to your kitchen, which is why I said "don't do this". Anyway, burning kitchens aside, Black Death was the catalyst for the production of newer higher quality oils, many of them man-made rather than mineral-based. Black death for the 21st century sludge There's a snappy new moniker for Black Death now, and it's called sludge. The cause is the same as Black Death and it seems to be regardless of maintenance or mileage. The chemical compounds in engine oils break down over time due to prolonged exposure to high temperatures and poor maintenance habits. When the oil oxidises, the additives separate from the oil and begin to chemically break down and solidify, leading to the baked-on oil deposits turning gelatinous, and that nasty compound is what is lovingly referred to nowadays as sludge. It's like black yoghurt. What doesn't help is that modern engines, due to packaging, have smaller sumps than in the "good old days" and so hold less oil. This means that the oil that is present in the engine can't hold as much crap (for want of a better word) and can lead to earlier chemical breakdown. The most common factor in sludge buildup is mineral oils combined with a lack of maintenance by the car owner combined with harsh driving conditions. But this isn't true in all cases. For some reason, a 2005 Consumer Reports article discovered that some engines from Audi, Chrysler, Saab, Toyota, and Volkswagen appear prone to sludge almost no matter how often the oil is changed.