Our first leg was from Corona to Mexicali, Mexico. We lined up for takeoff behind the slowest fixed-gear airplanes. We were the second to last to depart, and after starting our initial climb, Ralph opened our border crossing flight plan with the local FSS. At first, the Aztec performed as expected, but by the time we reached 4,500 feet, the right engine oil temperature hit the red line. Ralph leveled off and reduced MP to 20 inches on both engines. While this slowed us a bit, the right engine cooled enough in 10 minutes to resume our climb to 6,500 feet.