[79Chevypickup]

I own a 1979 Chevy Pickup (350ci) Longbed,
I have no idea how many miles or on it because the speedo
does not work and I dont know how long it hasnt worked...

So,

Which would you use and why?

Regular Oil.....
Synthetic Oil...
High millage Oil....

Please give me as much info as you can, anything will help...
and
Which ever you use, in your opinon what is the bonus about it?

Thanks in advance for all the input....