My 95 Legacy wagon has a little over 373,000 miles. As you might imagine, it burns a bit of oil, particularly when I rush to/from work by cruising at 75 - 80 on the interstate. I'll burn through two or three quarts of oil during a 3000 mile interval that way, but less than half that if I lighten my foot. My question is, when I'm pushing it and burning more oil, should I shorten or lengthen the interval, just replace the filter, etc? Part of me thinks that I'm effectively getting fresh oil and I should just change the filter. The other part of me thinks that's nuts and should change it all because there's probably bad stuff flowing through the system.

I wouldn't worry about it too much. As long as you're feeding it fresh oil when it needs it it should be fine. The filter will play a bigger part than losing/adding oil, and you do at least change that when it's supposed to be.

Yes, you're introducing more fresh oil, however, you also are collecting combustion byproducts and, probably more importantly, fuel from blowby.

 

the fuel probably reduces your viscosity.

 

Overall, you might consider just keeping a 'normal' interval and trying a higher viscosity or a brand made for high wear engines.

 

also, make darn certain your PCV valve and hoses are clean.

The 1" PCV hose coming up from the oil separator port on the back of the block and up to the PCV valve on the intake is often clogged with carbon gick. Try cleaning that hose and the nipple on the bock that it attaches to and putting a new subaru PCV valve in. It's worth making sure that it's not just sucking oil into the intake.

If it were me, I would go to a heaver weight oil, prolly 20W-50W, and drive on. It will burn less, and still lube a worn loose motor.

Check the PCV and vent hoses as suggested, and run a different oil. 20w50 may be too much, try 15w40. I run that in my 96 that has 296,000 miles and it's just fine. Mine doesn't burn oil , it just leaks it....

Check the PCV and vent hoses as suggested, and run a different oil. 20w50 may be too much, try 15w40. I run that in my 96 that has 296,000 miles and it's just fine. Mine doesn't burn oil , it just leaks it....

 

I have run the 20W50 oil during non winter months with no issues for several years now in my 99 OBW with the 2.5 motor. Like you said, it seems to leak less when using thicker oil. I have 194K miles on my odo.