Is it to late to use Synthetic oil in a engine with 55000 miles?

I'm not a mechanic / technician but I started using synthetic oil in my

vehicle when it had 51,000 miles back in 2001 . I would say as long as you

don't have any oil leaks or your engine is burning oil it's okay

The first thing I noticed was that the idle speed went up 200 RPM until the

computer made the necessary adjustment , also my highway mileage

improved slightly

You can use synthetic oil at any time. You can even mix synthetic with conventional oil. Just make sure you use the correct weight.