This souds like a 3HP Briggs & Stratton, in which case it's straight unleaded in the tank, and SAE30 4 stroke mower oil in the sump.
If it's definitely a 2-stroke, 50:1 mixture of unleaded and good synthetic 2-STROKE oil will be safe.
phil_saunders@bigfoot.com
Phil Saunders, December 2008