Low compression one cylinder

Getting a check engine light. Did a cylinder balance test, number three failed. Did a compression test, number three had 60 psi, number seven had 120 psi, and the rest had 140 psi. Just recently I accidentally put in 5w-20 oil when I usually use 5w-30. My mechanic said I should use 10w-30 or 10w-40 during the summertime. Before I go dumping money into a new engine I wanted to get a second opinion. I know you cannot see it, but the car is actually running okay, it is just showing a larger than usual fuel consumption lately. It was that and the light that caught my attention. The engine has about 70,000 on the rebuild, the chassis has about 200,000 on it. This is my daily driver. I love the car, I just do not want to spend money on a new engine if it might be something else.