What oil are you using in an older flat tappet motor?

[Nickelcharlie]

I have an 86 GMC with the original flat tappet motor. Should I be using an oil such as Rotella made for diesel motors or can I use a synthetic oil? I have also read once a flat tappet motor has been broken in correctly a synthetic oil can be used! So I would like to hear what you have been using successfully in your older motor for a long period of time?
My owners manual lists multi grade oils so I am thinking it is alright to be using any brand quality oil. Thanks for your input in advance!