Extending synthetic oil changes for short-city driving; analysis or indicators.

[NoraJL]

I drive a five-year-old sedan mostly on short city trips, and I've always followed the manufacturer's recommended oil change interval of every 10,000 miles with synthetic oil. However, a mechanic recently told me that with my driving habits, I should be changing it every 5,000 miles to prevent sludge buildup. I'm confused by the conflicting advice. For those who stick to longer synthetic intervals, do you use any specific oil analysis services to verify the oil's condition, or do you rely on other indicators to decide when a change is truly needed?