Another thing about oil and CB750.
One summer a few years ago was very hot here, often around 32C so I replaced Red Line 20W-50 with 20W-60 since oil pressure show rather low at lower rpm. I noticed that the starter gear started to slip rather often after that (CB900 DOHC starter). Clutch was still fine despite 90+whp.
Better when going back to 20W-50.
About synthetic and histories about more leak.
I'm sure that is when change to synthetic in an old engine that has long milage on old type of oil.
Compressed gaskets sealed by old oil residues. A newer type of oil/thinner oil might clean that burnt in stuff away and a leak might be born.
A new restore with fresh guides, seals, bores, pistons and gaskets another thing.