Two common narratives that I’m sure you’ve heard:
1. Some synthetics could cause deposits for some unknown reason – this is Mazda’s official line, and one that many independent rotary shops support
2. Many synthetics don’t burn easily enough, possibly accumulating and causing problems
I’m sure you’re also aware of the plethora of synthetic engine oils specifically for rotary engines, including the one Mazda sells under its own name specifically for the RX-8. And the large number of race teams (including Mazda's own), rotary tuners, and RX-8 owners that use synthetics exclusively and successfully.
Here’s my question: Could the "synthetics cause deposits" narrative have come from cases where the oil was too combustion-resistant and too dirty for the way the car was driven? In other words, might those issues have been solved by picking an cleaner-burning oil OR by driving harder?
Royal Purple quotes a range of 500º F to 1700º F for combustion temps in this car, which seems fairly plausible. The lower end of that range isn’t much higher than the flash points of some high-end synthetics. At those temps, I'd imagine those synthetics would burn, but not necessarily well, and not necessarily quickly. If they or their byproducts accumulate, they could not only burn poorly themselves, but also start to interfere with combustion of the fuel. All of that would increase deposit formation. Conventional oils might have a slightly easier time on average at those temps. Or for that matter, a semi- or full-syn with a relatively low flash point and high volatility, or really good self-cleaning, etc.
How plausible does this seem? What am I missing?
Those familiar w/ the RX-8 syn oil controversy...
- Thread starter d00df00d
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I always understood it as the synthetic oil would keep the seals so clean that leaks would form and seep oil into the rotor. If a conventional was used, the seals wouldn't be AS clean and would seep less oil into the rotor? It seems like there are a few theories out there...
Who cares it's a terrible engine design anyway and those that want to keep their vehicles a good long time shouldn't buy it.
The new PP GTL is a paraffin so what wrong with that? Does not the engine have a preoiler tank too or does it draw from the sump?
Originally Posted By: StevieC
Who cares it's a terrible engine design anyway and those that want to keep their vehicles a good long time shouldn't buy it.
Originally Posted By: ARCOgraphite
The new PP GTL is a paraffin so what wrong with that?
Originally Posted By: ARCOgraphite
Does not the engine have a preoiler tank too or does it draw from the sump?
Originally Posted By: StevieC
Originally Posted By: StevieC
Who cares it's a terrible engine design anyway and those that want to keep their vehicles a good long time shouldn't buy it.
Originally Posted By: d00df00d
Oh well. I knew what I was getting into. If I wanted to buy a car people understood, I'd probably have bought a soul-crushingly boring SUV or two.
It's ok to be an enthusiast though... 
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I wanted an RX8 in the worst way early 2009. But I needed a reliable daily, and something that could return decent fuel economy. Ended up with a Civic Si. Sometimes I still wish I would have experienced the RX8. The test drive sure was fun.
"A redline a day keeps a new engine at bay"
If you drive your rotary in a way that benefits it i.e. get it good and hot and rev the heck out of it, I think a synthetic oil would be a fine choice.
The rotaries that die the fastest are married to automatic transmissions and get granny driven - I think a synthetic oil would be detrimental in this case.
TCW-3 in the fuel is also a good thing.
Originally Posted By: d00df00d
If I wanted to buy a car people understood, I'd probably have bought a soul-crushingly boring SUV or two.
Originally Posted By: d00df00d
Originally Posted By: rooflessVW
The rotaries that die the fastest are married to automatic transmissions and get granny driven - I think a synthetic oil would be detrimental in this case.
Being a Rotary engine guy, Synthetics by american standard are not an issue. Current day synthetics in general shouldn't be an issue. But the oil that is provided by Mazda Japan for the rotary engines is a Group III oil which by their definition is not a synthetic. Here in the US, Group III oils are synthetic and almost all of our over the counter Synthetic oils are Group III. I personally would recommend using an xw30 or xw40 weight oil. In my RX7, I generally use 0w40 Castrol or 15w50 Mobil. It seems to really like both.
Sadly the real problem with oil in the Rx8 is the original spec is too thin. Tolerances are the exact same as older motors where xw40/xw50 oils were prescribed. It is also recommended to add 2 stroke to your gas. This helps keep the inside of the motor properly lubricated in the event there are issues with your OMP.
When I had my RX-7, I modified my pump with a kit from an aircraft company so that it injected straight 2-stroke oil from a tank. I actually used API-TC oils rather than TCW3.
Thinking of the rotors and seals is what got most rotaries. Running straight TC, the inside of my engine was clean as a whistle. Very few people believed that I ended up getting 161K out of a single build. Ultimately that engine kept on living in another car after my car got hit and totalled.
Dirty engine oil just isn't what should have been used there, but Mazda never had any faith in the public's ability or willingness to maintain a separate oil tank for injection.
Doing so, the rotary developed a well-established reputation as an amazing aircraft engine. Bad engines don't get that rep in the aviation industry, because engine failure can mean death, or at a minimum a lot of long and bad conversations with the FAA.
Originally Posted By: mazdamonky
the oil that is provided by Mazda Japan for the rotary engines is a Group III oil which by their definition is not a synthetic.
Stevie, I wouldn't say it's entirely horrible. The power density can be phenomenal, and do not have many of the limitations of piston-driven ICEs. That said, the inherent limitations and guaranteed oil consumption are negatives for a daily-driven vehicle.
If you're anal about maintenance, enjoy driving it hard, accept sub-standard mileage, and check oil levels on a frequent basis, there's nothing to completely steer you away from a rotary.
Originally Posted By: rooflessVW
Originally Posted By: d00df00d
Originally Posted By: rooflessVW
The rotaries that die the fastest are married to automatic transmissions and get granny driven - I think a synthetic oil would be detrimental in this case.
Originally Posted By: SubieRubyRoo
Stevie, I wouldn't say it's entirely horrible. The power density can be phenomenal, and do not have many of the limitations of piston-driven ICEs. That said, the inherent limitations and guaranteed oil consumption are negatives for a daily-driven vehicle.
If you're anal about maintenance, enjoy driving it hard, accept sub-standard mileage, and check oil levels on a frequent basis, there's nothing to completely steer you away from a rotary.
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