Is it OK to mix oil of different viscosity in a Wankel engine?

Question

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 higher 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 fast 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)?

Answer 1

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).

Answer 2

Someone mentioned rotaries and oil. 5w20 conventional is recommened most likely for fuel economy benefits but this is not the issue with durability. Rotary engines do infact use oil. Typically 1 quart every 3000 miles is normal. This is because the engine has a metering oil pump that directly injects oil into the rotor housings to lubricate the apex seals. The apex seals on the older rotaries were a different material than the RX-8 sintered seals. These sintered seals are what cause the problems with engine failures in RX-8's. Carbon (a natural by-product of combustion) hardens on the rotor housings and the seals can no longer 'seal' the combustion process. Before engine replacement on an engine with low compression Mazda recommends a de-carb procedure that helps remove the carbon build-up to restore compression. I've found that on an engine with marginally low compression this de-carb does sometimes work and can extend the life of a rotary. Also note that starting a rotary cold and shutting it off before it reaches operating temperature is a slow death to a rotary as flooding (raw fuel being dumped into the rotor housing) will cause excessive wear as the fuel washes away the oil it needs.

On the oil front, synthetics are not recommended by Mazda in rotaries as they were never extensively tested during development and the long term effects of apex seal durability is not known. Race teams do in-fact use synthetic oil but they also rebuild their engines regularly. Clean oil is the life blood of an engine and is especially important in a rotary, 3000 mile changes should be followed. The previous comment of running a rotary hard is correct. A good old fashioned highway tune-up, not lugging the engine in city traffic and allowing the engine to reach operating temperature before running it hard or shutting it off are your best defence against rotary engine failure. Contrary to popular belief rotaries can live a long life, 100,000 miles is not uncommon.

For someone not willing to live with the idiocyncracies and maintenance a rotary requires stay away. For those who know about and love rotaries nothing else will do.

20 year Mazda Master Technician :)

Answer 3

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.

Answer 4

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.

Answer 5

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

Answer 6

Rotaries aren't the most reliable engines. They're massive fun and they're smooth as silk, but because it's so smooth, you tend to not notice when you're over-revving. But even if you treat it well, the fact that you have a rotor scraping the inside of your engine means it MUST cause damage. The exhaust gases are also pretty hot for a non-turbo car and that could cause detonation.

Rotary engines burn oil. That's just the way it is. Check your levels every two to three weeks. I would definitely put in a type of oil that would provide lubrication at high RPM. Also, synthetics are very bad because a rotary engine, just like a 2-stroke engine actually burns off oil and synthetics resist being burned off. Some folks actually mix their oil with 2-stroke oil. Ask a specialist rotary performance shop about this.

Second, when they are revved to high RPMs the apex seals may damage the inside of the engine after a while. This damage is referred to as chatter marks. This is caused by carbon build-up (see the next point). To minimize this damage, you need to use an oil that will lubricate your engine sufficiently when revving as high as a rotary does.

Third, it is very important to rev the car to its limit at least once a week to burn off excess carbon build-up. Or just go for a nice, spirited drive. Allow the car to warm up properly beforehand and cool down after. This is also a good idea for turbo owners. I don't know if it's as important in the RX8 as it was with the RX2 and RX7, but rather safe than sorry.

And finally, keep in mind that the rotary engine is a high-performance engine. It isn't intended to be used at slow speeds, crawling through traffic. Overheating a Rotary is VERY bad.