Depends on the compression.
All the manufacturers recommended using premium fuel when leaded went away
to help prevent detonation.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
"John Wentworth" wrote in message
...
GC wrote:
So you think I would be safe using Premium Unleaded (the highest octane
petrol available in AUST) plus my standard mixture of Two Stroke oil?
Would I have to drain out my existing tank completly before mixing the
leaded petrol with the Unleaded?

I have been using unleaded regular fuel in an old Evinrude outboard for
many years, when leaded regular was no longer available I simply added
unleaded to the tank. No problems.
There is no harm in using premium unleaded, but does your motor need that
octane rating?