the 3.0 today is the same engine offered 30 years ago (no design changes to the GM motor since 1991 when they went to a 1-piece RMS and the same piston diameter as the 350). prior to that, the similar motor had been around for nearly 35 years unchanged. it gets its roots from the 1962 Chevrolet 153 (the 230cubic inch inline 6 banger with two cylinders removed) the 3.0 was a bored and stroked version of it.
over the years, the oil specification has been SAE 30, SAE 40, 20W40, 20W50 everything from straight dino oil to straight synthetic and everything in-between
I ran rotella 15W40 diesel oil in my 3.0 for years (because it exceeds what the motor was designed for, and is cheap as dirt)
the motor is a dinosaur of a motor from a design point, it is not a power house, and the bearing clearances are sloppy, so run a good quality oil in any of the blends in any of the mentioned viscosities and go boating
