Hi, 

I have an '02 Outback Limited, 5spd manual and 4cyl with 173k miles. I've owned it since August 2020 and it had 163k when I bought it. It doesn't appear to leak oil (no slicks under the car), I've never seen blue smoke (it does apparently smoke white on occasion, although I can't see it from the driver's seat), and I never smell anything burning or any other unusual symptoms. But it loses oil somehow. When I got my first oil change in that car, the shop told me that the car was empty of oil, completely. No odd noises, it didn't overheat, and I'd been checking the oil 1-2x/week and it was always full. I'd actually checked it 2 or 3 days before that first oil change and it was totally fine, so I thought perhaps the shop was lying even though they didn't try to sell me anything else. I kept checking the oil, even more regularly and established a pattern that the oil level would be consistently fine for a while and then all of a sudden be empty. No symptoms of loss that I noticed, the dipstick would just be dry or nearly dry and this happened around every 800 miles, maybe longer at first (I didn't measure the mileage interval until a couple of months ago). I finally took it in to a different shop for diagnosis because I'm planning on a road trip. They did a compression test and then determined a leaky valve cover gasket. They replaced it and did an oil/filter change, and 900 miles later the dipstick had not one drop of oil on it. Any suggestions? By far the majority of driving I do is highway, 5th gear in that car is 3500rpm at 75 mph. Car overheated once in August due to a faulty relay, but oil level was completely fine then. No other issues, no warning lights, no other overheating, no anything else. Runs perfectly except for the oil loss.

I check the oil on a flat surface when the engine is cool. I pull out the dipstick, wipe it clean, reinsert, take it out and look at the level. Also, I've always used 5W-30 oil, per recommendation of the owner's manual.

Thanks for any help.