Yes, but keep in mind that 5w30 thins out at higher temperatures vs. 10w30. i.e. 5w30 is better for cooler climates than 10w30 respectively. Theoretically you could see a mpg increase with the thinner oil comparatively, just don't sacrifice proper lubrication to save a few bucks.
In my '95 2.5l it never lost any oil when I ran 10w30. When I tried 5w30 it did go through some. I have a 99k engine. the oil wasn't being burnt, so I wasn't losing it quickly, I just needed to top it off after a few months. I probably had to add 1/2 quart.
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Yes you can use 5W20 oil instead of 5W30. One is just thicker then the other. The 5W20 is better in the winter and the 5W30 is better in the summer.
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