corolla '78 motor oil

[crdiscoverer]

I change my car's motor oil every 3000 Km (about 1850 miles), but just after half of that distance the oil becomes dirt black. Some time ago I used an additive to clean the engine with the next oil change, thinking that it could be the solution... but no. I don't know what's wrong, my car is a KE30 '78 Corolla, but the engine performs as new (it's an amazingly reliable engine with just very little maintenance). The car is working fine, but I want to know if there's something to do about it, I use High Mileage Vehicle oil (not syntetic).