Look at me digging up old posts...yeeehaaarrgh!
The rule of thumb for most engines from japan is whether they we're manufactured before or after 1975.
Japan introduce unleaded petroleum in 1975 and pretty much all manufacturers either replaced their engines or introduced hardened valve seats to compensate for the lack of lead.
If you're running a pre 75 head on unleaded then you can possibly be doing damage and not realising it until it's too late. You can switch to unleaded for a while because the lead you we're running will stay partly embedded in to the valve seats but will wear off over a short period of time.
So basically you could have 2 motors of the same capacity and model but one was produced before 75 and one after 75, you could find different materials in the head therefore requiring different fuels.
All that said, I'm not a mechanic so if anyone has better info or thinks I'm wrong, please post your expertise