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[Z4 DCT]

WHAT ELSE HAPPENS WHEN I CHANGE THE OIL THEN?
Engines pump about 10,000 litres of air for every litre of fuel consumed, and along with all that air, they suck in plenty of dirt and grit. A good air filter will stop everything bigger than a micron in diameter - everything smaller mostly just floats around harmlessly in the 0.001inch minimum thickness oil films that separate all the moving parts. Despite all of this, there will always be submicron particles that get in and there will be places in the engines oilways where they will gather. Every time you empty the oil from your sump, you're also draining this fine grit with it.
CHECKING THE OIL IN YOUR ENGINE, AND TOPPING UP.
Where the oil is in a typical engine
Note that this section only applies to wet sump engines - the type found in most consumer vehicles. For more info on sump types, see Wet sumps vs. dry sumps below.
To a lot of people, this little section could be categorised by the rearranging the words "granny eggs teaching suck your to". But you'd be surprised by the number of people that don't know how to do even this basic task. When checking the level of oil in the engine, the car should be on a level surface, and should be relatively cold. I've run into several people lately who insist in keeping the crankcase topped off completely, and they invariably check the dipstick just after shutting down the engine. Checking the oil this way results in an erroneous reading because a quantity of oil (usually about half a litre) is still confined in the oilways and passages (galleries) of the engine, and takes some time to drain back into the crankcase. (On the image, the blue areas are where oil is likely to still be running back down to the sump). On seeing what appears to be an abnormally low level on the dipstick, these people then add more oil to the oil filler at the top of the engine. The oilways and passages all empty, and suddenly the engine becomes over-filled with oil, going way above the 'MAX' mark on the dipstick.
WHAT HAPPENS WHEN AN ENGINE IS OVERFILLED WITH OIL?
So you topped up the engine when it was warm after getting a faulty dipstick reading, or you put too much oil in when you changed it yourself. What's the worst that could happen? The problem with this is that the next time the engine is run, the windage in the crankcase and other pressures generated by the oil pump, etc. place a great strain on the seal on the rear main bearing.
Eventually, often much sooner than the ordinary man in the street might expect, the rear main bearing seal ruptures, and the engine becomes a 'leaker'. If you've got a manual gearbox, this means one thing: this oil goes right onto the flywheel and the face of the clutch disc. A lubricated clutch is A Bad Thing. If this still goes unnoticed, the front seal is the next to go, and the engine then becomes a 'gusher' (or to be more colourful, it starts pissing oil all over the place). As well as smothering the clutch with oil from the rear, the oil now coming from the front leak will be neatly distributed about the engine bay as it hits the front pulley - often propelling it out as far as the brake discs. At the same time as this Hollywood disaster movie is unfolding outside the engine, things aren't working out any better on the inside. As you can see from the diagram, the correct oil level is really close to the rotating crank. Overfilling will mean the crank dips into the oil and churns it into a froth. Froth is good on certain types of coffee but not good in an engine. The mixture of aerated oil will be forced into the bearings and in case you didn't know, air is not a lubricant. Typically this means that bearing damage will follow quite rapidly, especially if you are driving on a motorway. You'll know bearing damage when you get it. The engine smells like a garage mechanic cooking over an open flame and the noise coming from the engine is the sort of thing you'd normally hear in vaudeville plays when a piano is pushed down a flight of stairs. As if that all wasn't bad enough, the excess oil gets thrown up into the piston bores where the piston rings have a hard time coping with the excess oil and pressure. It gets into the combustion chamber and some of it will get out into the exhaust system unburned resulting in a nice patina of oil all over the putf8um surfaces of your catalytic converter. This renders it utterly useless for good.
Well, you did ask.
SO WHAT'S THE BEST WAY TO CHECK THE OIL LEVEL?
If your engine is cold (for example it has been parked overnight) you can check the oil level right away. The oil will have had time to settle back into the sump. Just make sure the car is level before you do. If the engine is warm or hot (after you've been driving) then you should wait for 30 minutes or so to let as much oil as possible drain back into the sump. Checking it first thing the next morning is ideal.
It's worth pointing out that you should double-check your owner's manual too - some cars, like I the '92 Porcshe Carrera, require that the oil is checked while the engine is running and the oil is at temperature.
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WET SUMPS VS. DRY SUMPS.
Different types of oil sump
Almost all passenger cars, trucks and SUVs use what's called a wet sump system. If you look at the diagram above you can see the sump (or oil pan) is the lowest part of the engine. In a wet sump system, excess oil drains back into the sump when it has passed through the engine, and the oil pump then sucks it out of the sump and pumps it back to the top of the engine. The advantage of a wet sump is that it's cost effective to build and maintain and it makes oil-checking easy for the average driver. The disadvantage is that cornering and braking can cause the oil to slosh around in the sump. This can cause the oil to not cover the oil pump pickup tube, which could starve the top end of oil, or it could get deep enough in a severe cornering maneuver to bog-down the crank, which is A Bad Thing. To counter these problems, a lot of wet sumps have baffles in them to stop the oil moving around so much, and for your average road-going consumer-level vehicle, this is a fine compromise.
DRY SUMPS
When it comes to racing vehicles, wet sumps simply have too many disadvantages. Instead, race engines typically use a dry sump. As its name implies, the sump of the engine is dry - it never fills with oil. In a wet-sump system, the sump has to be large enough to accommodate all the oil from the engine when it is turned off. In a dry sump system, that requirement is gone so the sump can be much much smaller. (In the image on the right, the right-most sump is representative of a dry sump). A smaller sump means the engine can be mounted lower down in the vehicle, which in turn lowers the centre of gravity - great for racing. So how can this be? Well a dry sump system uses a remote oil reservoir or tank, and a either a second oil pump, or a single multi-stage pump. In a double pump system, one oil pump works just like a wet sump - it distributes oil to the top end of the engine, but it pulls the oil from the reservoir instead of the sump. The second pump scavenges the oil from the sump and returns it to the reservoir. In a single pump system, one pump is either a three- or four-stage pump. It has multiple circuits running off the same pump to pressurise the engine and scavenge oil back from the sump. The pumps typically don't run off the crank-driven belts so no engine power is sapped in driving them. The remote tank or reservoir can be pretty much any size you like and be mounted anywhere in the vehicle (usually low down again for centre of gravity reasons). There isn't oil sloshing around in the sump so you don't run the risk of bogging down the crank. For all these reasons, dry sumps are considered to be safer and far more dependable than their wet counterparts. So if it's that much better, why don't you find this system in consumer vehicles? Simple. The increased weight, complexity and cost of having larger or more pumps and a remote reservoir with all the additional high pressure oil lines involved. For a racing team, this isn't an issue, but for Toyota or Ford, adding that sort of cost and complexity to their passenger vehicles is just a no-go.