What oil?

Hey-
I have a '68 Barracuda Formula-S 340.
All original matching #'s.
123,000 on motor.
Want to change oil, but don't know what grade/weight to use.
I'm in Long Island, NY and car is stored for winter.
Also I've seen this oil from quaker state:
Quaker State® Higher Mileage Engine™ with Slick 50® http://www.quakerstate.com/products_motor_oil.aspx
What do you think?

Thanks, Rick