Oil Type

There is rationale for the lighter weigths at least: The start up oiling will be faster, and the oil pump gear in the /6 is on the small/weak side so lighter oils present less pumping load. (Check out the oil pump article on www.slantsix.org). So be cautious with the oil weight and keep it on the low end.

Certainly any good quality oil will work. The need for the higher zinc levels on these low lift cams is arguable.

I just prefer synthetic for the lower cylinder and ring wear that you will definitely have, the superior bearing protection in case of a loss of oiling, and the almost no-breakdown of the synthetic oils.

The leak that the OP has will be easy to fix. When you pull the vavle cover, adjust the valves.

Also check for sludging in the engine. If you have a lot of sludge, it might be wiser to run a high detergent motor oil for several quick oil changes to try to dissolve some of the sludge out. Running it repeatedly to where it gets hot at highway speeds will help dissolve some of the sludge and change out after say 500 miles or so. Change the filter each time. There are 'recipes' for cleaning sludge; can't say if you should try that or not.