I have a Mazda RX-8 (with a Wankel rotary engine). Mazda advises North American owners to use 5W-20 oil. The engine was just replaced 200 miles ago by the dealer so it currently has 5W-20 in it.
Due to the large number of engine failures, although the exact cause is under dispute, many owners have come to agree that Mazda most likely advises 5W-20 oil for gas mileage and not for engine protection. They suggest a thicker oil such as 5W-30 is better protection for high RPMs.
The RX-8 burns oil at a pretty faste rate; I will be adding a quart of oil in another 500-1000 miles. Should I wait to switch to 5W-30 when I change my oil, or is it OK to refill with 5W-30 (and mix with the existing oil)?