I have changed my car and truck over to synthetic oil. I have been running them around 10,000 miles between changes or until the oil is just too dirty looking to justify leaving in.  I have heard that synthetic oil doesn't lose its viscosity. I know that when I drain regular oil with more than 5000 miles on it, it runs out like water. Synthetic oil, even after 10,000 miles still pours like it did new. Is there any way to measure viscosity to see if it really is still usable?

 I've been running my car on Mobil1 Synthetic and changing it every 5,000 miles.  Nice to hear your visual analysis is showing good viscosity up to 2x that.  I have not read anything specific but I'm sure the Jiffy Lube's and oil manufacturers would prefer we change our oil more frequently so they can make more profit.

 One thing about "synthetic" motor oil: A court case recently said that pretty much anything can be called synthetic oil. The oil company websites are little help.

Just don't go to a higher viscosity oil. That's a problem.