Dan.

In the past, Lycoming has recommended straight mineral oil (single weight)
for break-in. As I understand it, it's important to use an oil with no
friction reducing additives or the break-in could be delayed or even stopped
entirely. To properly break-in, parts of the engine (like ring-to-cylinder
walls) must break thru the oil barrier and actually wear some.

When I received my engine from the factory (a factory re-man O-235), the
instructions specifically mandated such oil. I used Shell 100 for about the
first 30 hours (changed after the first 5 hours) or so, until oil
consumption dropped to around1 qt. in 15 hours. Consumption has remained at
that level for over 700 hours so the technique works just fine.

I understand that there are some engines (some big Continentals, I think,
but maybe more) that can use multi-weight oils - but for a Lyc 320, I would
go with a straight mineral oil like Shell 100. It's a tradition that works.

Good luck,

Dan Schaefer