Before switching to a remote filter and oil cooler system I found Torco blends were very good and normally in the 10w40 range.
http://www.torcousa.com/torco_product/t4.html .
But nothing takes the place of lost metal due to wear, not even an oil cooler.
Teflon additives, synthetic blends nothing will put metal back. Heat equals wear and 40 year old parts have wear which will result in friction and more heat with an ultimate failure of parts. Because most of the bearings in the Honda 600 engine/transmission are roller or ball bearing system wear is more focused on bearing cages and thrust washer/bearing units. Other weak links would be the cam chain and oil pump rod. Some would say pistons and rings also are a weak link, but only after pour oil flow from an out of time engine where the cam chain has stretched and the eccentric the oil pump rod rides on has warn to the point where the oil pump is giving a reduced flow of oil. The better question is not what oil, but is my engine warn beyond usable limits?