Is it necessary to continue using 5w-20 grade motor oil even after 90k miles?...

 

You should continue using 5W-20 oil no matter the mileage unless you're having a problem with oil consumption.

 

You might get by with 5W30 since most shops like jiffy lube and others use 5W30 as a standard oil change. 90K is not that much to worry about, if it had regular oil changes you should be fine.

 

I run 5w30 with 253,xxx miles. Only oil consumption is a 'seep' issue near the dizzy and stripped out (thanks quiky lube) oil pan plug.

In the end, it's your car, do what you want. I just told you how my old beat up Accord handles the 5w30.