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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)?

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Even if you change viscosities at the oil change time, there's usually somewhere around a half quart of the old stuff that you won't get out (check the manual for dry fill quantity versus oil change quantity). So, you're always mixing when you make a switch... – Brian Knoblauch Feb 22 '12 at 19:34
"The engine was just replaced" - did you mean to say the engine was replaced? Or the oil? – Bob Cross♦ Mar 9 '12 at 20:31
@BobCross New engine. I'm intending to switch to 5W-30 as better protection for the new engine. – NickC Mar 9 '12 at 23:19

2 Answers

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My guess is the switch to higher viscosity is not providing better protection but reduces the amount of oil lost in the combustion process.Because it is thicker less of it blows by into the exhaust.This indirectly lessens the time the engine is running with less than optimal oil volume.The potential down side is at lower ambient temps.the oil will take longer to circulate through the motor on a cold start.Also a slight decrease in mpg as stated as the engine works harder to move the thicker oil.Mixing viscosity won't cause any harm.My theory has always been that 5w30 with 1000 miles on it is no thicker than 5w20 with 0miles on it.

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Mixing oil viscosity should be fine. It isn't advised to mix oil brands though.

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Interesting - can you elaborate on the brands thing? Like, say I'm in a pinch (realized I'm short on oil at a gas station and don't have extra in my trunk). How bad is it? – NickC Mar 9 '12 at 20:29
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@NickC - mixing brands of oil isn't advised simply because of the different additives used in each brand of oil. When mixed they may not work as well as desired. That being said, it is better to maintain proper oil levels than to worry about how efficient it is at keeping your engine clean. It is explained a little more here – Tim Mar 9 '12 at 22:16

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