I am not sure but it seems that tensioner rail needs removal of cylinder head.
An upper guiding rail and a lower guiding rail can be replaced without removal of cylinder head.

Of course not. I do not want to remove cylinder head. I am going to do an oil analysis to see whether my engine mixes fluids.

I get 25 mpg compared to 30 mpg last summer (with almost the same highway and city ratio). A dealer tech said timing chain (even if it were stretched) might not be responsible for 20% decrease in mpg.
He said it might not even be worth measuring timing chain stretch or worrying about the chain.

While an indi recommended immediately replacing chain without even measuring the stretch on my 230 000 mile car.