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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
    
Is losing 1-qt per 500-1000 miles common in rotary engines? –  jp2code Jan 21 at 19:08
    
@jp2code It can burn oil at a rate of up to 1 liter per 1000 miles, I believe. I've seen it at that rate or slower. Not exactly sure what explains the variation, but I think it's a combination of ambient air temp and average RPMs. –  NickC Jan 22 at 6:53

4 Answers 4

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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Realizing this is a late answer to this question ... When you pour different weights of oil together, they DO NOT MIX. They will separate out after you shut your engine off. This won't do well for your engine.

Putting different BRANDS together of the same weight will not have ill effects on your engine, but usually once you pick a brand, stick with it. Detergent packages in the different oils may not mix well together, which may cause unexpected results.

All this said, stick with the same weight of oil, as in, don't mix weights ... Wait until you change the oil to change viscosity. If it's an emergency where you don't have a choice, any oil is better than no oil (within reason ... don't use gear oil in your crank case).

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Wow, another down vote without a comment. –  Paulster2 Jan 21 at 17:11
    
Join the club, man. –  Juann Strauss Jan 22 at 14:59

You can mix oils synthetic or mineral as well as different viscosities. Only worry is for warrenty other than that you just changing the viscosity. Median range oils will not seperate simply beacuse they are synthetic vs mineral. In actuality most "synthetic" oils are actually mineral oils that meet the spec of synthetics and therfore are allowed by our governments to be labelled synthetic. The only true synthetic oil I know of is Mobil delvac and I have been working in the oil industry for 15 years.

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If that's the case, then what the heck does synthetic blend actually mean? –  Robert S. Barnes Dec 4 at 8:17

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
    
It's also not advised to mix different brands of antifreeze because if you're really unlucky, two types may react and form a kind of gel, rendering it utterly useless. –  Juann Strauss Jan 21 at 8:57

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