When do I need an oil change?

dodge guy wrote:

05 F350 with a small dump bed, 5.4L gas with 92k miles. engine is noisy when warm. customer said he changes the oil (he thinks) every 8-10k miles :S . inside the engine is black and there is some sludge, oil is very dirty! I will get a pic tomorrow. truck doesn`t look to be beat on and in decent shape. just no regular maintenance.

I know I`m beating my head against the wall here, but for those that wonder about changing your oil sooner and keep asking why should I, well here is 6000 ($$$$$) reasons to spend $30 on an oil change every 3-5k miles! and keep up on other maintenance too.

My brother in law has a similar method of oil changes. He came over for a kids birthday party and when he pulled in the driveway I thought his engine sounded strange (we both had the same car at the time) I checked the oil and it wasn't even on the dipstick it took 4.5L of oil to top it off it takes 4.8L to change it. I asked him when he last changed it and he said he JUST had it changed. A few more questions and JUST changed was over a year ago.

Judgement is very important when changing your oil, if you change based on the computer and your oils black you should consider changing it more regularly if it seems to be good continue to flooow it. I normally change it when it is at 25% usually around 5000 miles But I have run it down to 3% once.