hi. i have honda civic lx year 2000 with 233000 miles write now on odometer.i'am using 10w-30 hm mobil 1 synthethic for about 30000 miles, i switch from regular to synthetic.i'am not shure should i stay with 10w-30 or should i switch to 10w-40 or 5w-40, because is thicker than 10w-30 with this mileage.i did a head job on motor at 220000 miles, it was heating, and now everything is fine. i was thinking should i come back to dino oil, because i can't see much difference between synthetic and dino on my car, it's get dirty around 4000 miles synthetic oil. i don't know why some companies say for e.g mobil 1 synthetic extended performance change around 15000 miles, that's balooney for me, i bought that oil ,but before i pour inside engine i did motor flush+ i add 3 liters of some off brand synthetic oil to clean the engine and than i add that oil , and get dirty around 4000-5000 miles. i also try castrol edge full synthetic same thing they last about same 4-5k miles. my car it doesn't burn oil, only if if black completly than burning. if you can give me some advice, thanks.........