My 91 LX just hit 287k miles. Within the last 15-20k miles I have switched to full synthetic 10w30, and doing routine changes every 3k, and the oil is always dirty, and black. I recently moved, and my driving routine has changed. I drive maybe 3k miles a month now, and a good majority of it is straight highway. I do accelerate faster than average, but I don't abuse it like one would think.

I'm thinking changing it every 3k miles is a colossal waste of money, considering I can probably extend it to 5k miles.

I just feel like if I don't change it every 3k miles, regardless of using full synthetic, that I'm 'hurting' the engine.

Is it safe to let it go longer than 3k?

 

The reason it gets black is because of blow-by which is completely normal. If you're using fully synthetic you can easily go 10k no problem. You should get an extended oil filter as well. Then change them both at 10k.

 

3k miles was for carbureted engines. Unburnt gas would condense on the cylinder wall after the engine was shut off and run past the rings into the oil. After a while the oil gets diluted and things. Engine flooding sped the process up. It's a nearly nonexistent problem with computerized fuel injection engines. 10 thousand miles between changes seems long to me. I change mine every six months. I feel it's more important to keep a routine of it than to count mileage.

 

If your oil is turning black quikly...

Verify the PCV system is working properly. Fresh air intake and PCV valve are correctly attached.
Check your air filter is not plugged.
Verify the compression is within specs.
Verify overall condition of the engine. Air/Spark/Fuel.
Use seafoam or some ATF in the engine oil to break down and remove any crud in the engine.
If the oil in the past was cheap or didn't use detergents(Penzoil) then your oil will be blacker quicker.
Between oil changes, while the engine is hot, I drain the engine oil. Refill the engine with atf. Let it idle for 10mins, NO LOAD! Drain that, and then replace the filter and put in the new oil. ATF usually comes out pretty gloppy on older/high mileage/poor maintenance engines.

 

The single best maintenance you can do to your engine is an oil/filter change, I do mine every 2500 km and every spring, a lot yes, but worth it in my opinion, I do not use synthetic, that would be a waste of money. 94