I have an 08 forester with the common p0026 and p0028 check engine error. I have replaced both oil control valves and the solenoid on the passenger side. After running the car for a few miles, the CEL comes back on.  Just wondering what the next step would be to fix this issue. The car sat for a year so I'm wondering if I'll have to change the oil a few times to get any sludge in the oil out which may be causing the CEL?

Sounds like you've done all the common fixes. I've hsad these codes on my 2008 Forester and the 2006 I used to have. For me, the oil control valve did the trick. I believe they are sensitive to oil levels, so an oil or change or two might help

Change the oil filter. These codes pop up due to old or problematic oil filters. I’m guessing it’s never been changed?

Is the oil pan dented?  

If the filter and pan are fine check the oil pressure with a gauge and see what you got.