Hardware recommendations for heavy beam mounted to wall
I think your choice of 5/16&quot; size lag screws, 2-1/2&quot; long, is fine.  I would say 3/8&quot; is overkill but if you just like the look of the bigger fastener, it would be fine.
I'd actually use something a little smaller, but longer - 1/4&quot; size, and 3&quot; long. (And washers, whatever size you use, I'd use washers.)  One of those is adequate to support 90#.  I'd also drill a pilot hole, 3/16&quot; bit works well for 1/4&quot; fasteners in most wood.