"I didnt know how often one should do oil change-I just changed mine-its been over 8,000 miles since the last change. Yes, what was left in there was really dark, the only way I know this is because the new oil is much lighter on the dipstick :-) I guess I was lucky I didnt cause damage."
If you drive under normal conditions change oil and filter every 6 to 7 thousand miles. That means very little city driving idling the engine and not much on dusty, dirt roads on steep hills or pulling a trailer. Those are considered HARSH conditions so change every 5 thousand miles and preferably four thousand. I drive in easy conditions so change air filter every 15K miles, Don't forget the fuel filter should be changed every 25 to 30K miles.
One thing for sure, oil and filters are a whole lot cheaper than an engine overhaul. Also the transmission fluid should be changed, (or flushed), at 50K miles. Very important not to neglect the tranny because overhauls are very expensive. Look in the owners manual and it will give you a prescribed maintenance schedule, so adhere to it and you should be good to go.
Color of oil: When I change mine it's pretty clean for the first 1500 miles then it starts getting dirtier and darker which is what high detergent oil is suppose to do. Like Willyum said, that way you know it is doing it's job keeping inside of the engine clean.
Old tranny fluid will be dark reddish-brownish while fresh fluid is a light reddish-pink and if it smells burnt that indicates it's time for a fluid change. Don't forget the cooling system because that is one of the most important things and the most neglected. Maybe it's overkill but I drain and flush then replace the anti freeze every two years. Brake fluid and power steering fluid the same.