The PH/D-series Honda engines should have no problem with fully-synthetic oil. I ran a D15B7 myself for six years (it was a late-90s car) and it never had any problems with the stuff. The D-series engines soldiered on in Honda engine bays for a good 20 years with not much change in the basic structure.

If you do want a multigrade oil, try 15W-40 or 10W-40 for extra protection for your old PH16 engine.