When to change the oil in my engine?
1 - 3 months
If you do a lot of short drives around town then the 3 month rule will probably be best as you are not heating the oil up enough to help keep it clean.
2 - 3000 miles
Like the 3 month rule the 3000 mile rule has been around for a while. If you drive on the freeway a lot then you could easily double this especially with the new synthetic oils available now. If you are doing just general driving in town and a little bit of freeway then the 3000 mile rule probably applies.
3 - Freeway Driving
Engines love the freeway driving of being run at a constant RPM for miles on end. As stated before if you do this a lot then you could easily go for 6000 miles between oil changes if you are using a good oil.
4 - Use your nose
Smell the end of the dipstick. If the oil smells burnt or like gasoline then it is time to change.
5 - Use your eyes
On level ground pull out the dipstick and clean it off, stick it back in and then pull it out and check the level. If it is low now would be a good time to consider an oil change but keep track of how often it gets low as you may have a leak. Also look for the oil being dark - (black in color) as this suggests it is very dirty.
6 - Car gets flooded
If your car ever gets flooded then all the oils/fluids/filters need to be changed immediately before the engine is run again.
Don't Forget the Filter
Filters are important for keeping oil clean. When you change the oil you should change the filter.
How does oil get dirty?
1 - Gases from the engine
As the engine compresses gases and exhausts gases some get by the pistons and into the sump where the oil is. Over time these gases get absorbed by the car oil which changes the chemical properties making it not so good at doing what it is meant to do.
2 - Lubrication
Oil is everywhere in the engine lubricating parts so that they run smoothly. As parts wear little microscopic bits end up in the oil and overtime they can form a paste which ain''t no good for your engine.
3 - Overheating
If the oil gets too hot it starts to burn and form hard carbon particles. Remember Diamond is just hard Carbon and that is no pillow. These hard carbon particles will cause wear in no time if allowed to remain in your engine.
4 - Water & Humidity
Water vapor from hot summer evenings and snowy winter nights can end up working it way into the oil. This water vapor can be heated out by driving the car until the engine gage shows warm however short trips around town do not achieve this so the water which is bad for the engine remains in the oil.
5 - Dusty, Dirty or Sandy Environment
Even the best seals and filters in the world are not able to keep all the dust, dirt and sand out of the engine. An engine needs to breathe, expel gasses and be fueled all of which allow dust, dirt and sand to get into the engine over time. This ends up in the oil and eventually causes the oil not to be as effective as it once was.