Oil slowly breaks down over time. This is an inescapable fact. Normally, you can tell bad oil easily by its black appearance. Just because oil is not carbon-polluted doesn't mean it's still good. It can break down, or have its additives corrupted by metallic components from your engine. The best way to find the ACTUAL condition of your oil is to have it analyzed. Only then will you TRULY know if it can go farther or not. From the sound of it, you're unsure - if you drive it every day, change it, and analyze what you took out - you'll know if you can take the next change further safely AND you'll have safety of mind while you wait for the results.

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Back in the saddle - 1972 280SE 4.5