Okay, I don't really have a million dollars (surprise, surprise), but I could really use your combined expertise on this one.

Engine is a normally aspirated B230F w/191,000 miles. Engine has been serviced religiously until recently (see below) and has never been problematic. Fuel economy is excellent (26-27mpg) and she goes like a scalded cat when I floor 'er. Doesn't burn a drop of oil. All in all, incredibly healthy for a 16 year old motor with nearly 200,000 miles on the ticker.

But, in the last couple of months, she has began to sound sickly. I'm not exactly sure how to describe this, but she just doesn't sound normal. The old girl "ticks" at idle speed (can't determine if it's my crusty old injectors or the valvetrain) and has developed a hallow, metallic knocking that is most audible between idle and about 1500rpm. I don't know if this is piston slap or a more serious problem, but I tend to lean towards the latter as it doesn't stop once the engine has reached operating temperature and is, in fact, more audible when the engine has been running for a few hours.

Here comes my admission of guilt...I just changed the oil (syntec 10w30)after almost 8,000 miles, and it looked way overdue. Nice 'n dark (but not black) and dirty. I went 6,000 mi. time before that. I figured that since I'm running a (semi-)synthetic, I could get away with a longer change interval, but now I'm afraid that I might have gone too far. On the other hand, the owner's manual suggests a 7,500 mile interval, so maybe my paranoia is unwarranted. I plan to change it at 3,000 this time to be on the safe side.

Have I caused irreparable damage, or made an existing condition worse, by changing the oil at longer intervals? Any ideas about the source of the knocking?

Thanks in advance, folks! ;-)

Forever indebted to the BrickBoard,