Prius (2004-09) :: Engine Knocks In Summer Under Heavy Load

I am the original owner of a 2006 Prius with 265k miles. I use Mobil 5-30 synthetic oil and I drive a steep hill every day and the engine knocks in the summer afternoons when I use cheap 87 Octane gasoline from Safeway or Costco. If I use better 87 octane gasoline (Chevron, Shell) generally there is less knocking. Also during the winter months no knocking even with cheap gasoline.

During summer I can sometimes make the knocking go away by accelerating or decelerating. The knocking is intermittent and generally occurs only during the steepest parts of the commute. The engine might knock for 30 seconds to 60 seconds before its goes away as I try to change the engine RPM.

Unfortunately I am burning some oil at a rate of approximately half a quart every 5k miles. Generally I watch the oil level, but there have been cases where the oil level has fallen below the lowest mark on the dip stick so I suspect I have some carbon build up.

I've read posts where others have used additives like gumout regane and I've read posts that say not to use additives.