my car seems to be losing oil and there are no leaks. what can be the issue? on 1996 Nissan Sentra

This car has 136,000 miles and I changed everything that comes along with a tuneup.

Asked by for the 1996 Nissan Sentra
You must be burning oil because of sticking or worn piston rings or loose valve guides and/or guide seals. Possible that PCV is worn out and you could lose a little oil that way too. You didn't mention how much oil you use.