Thanks Biglow!! my valve seal issue

super thanks!!
You are really an expert.
So how to evaluate how mush consumed for engine oil for 3000 miles or 3 months?
Go to the same shop I did the last oil change? They should know how much they put in, and then compare with the current one if it's just 3000 miles already, do they?
Last thing, how to test or know if engine performance has變化 or not.
I did not do engine oil regularly, sometimes wait till 6000+ miles and then to do it, could it be the root cause of the issue?