(Figured necro-bumping this thread would be less rude than posting a new one on this issue. Mods, please feel free to move as appropriate.)

Question: Anyone else think maybe the "synthetics cause deposits" narrative comes from cases where the oil was too combustion-resistant for the way the car was driven? I.e., it would have been solved by picking an easier-burning oil OR by driving harder? I'm sure people have thought about this, but I don't think I've seen it discussed in quite these terms.

Royal Purple quotes a range of 500º F to 1700º F for combustion temps, which seems fairly plausible (thanks for confirming, Loki). The lower end of that range isn’t much higher than the flash points of some high-end synthetics. At that temp, those high-end synthetics will burn, but not necessarily well, and not necessarily quickly. If they 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, high volatility, etc.

Maybe that's why race teams, some rotary tuners, and many people on this forum report zero issues with high-end synthetics, while MNAO and certain other rotary shops insist on the "synthetics cause deposits" narrative for street cars. The former group are running "hard-to-burn" synthetics, but they're driving in a way that'll burn them efficiently. Other folks might just be throwing Mobil 1 in there because it seems like a good idea, but never spending enough time with high enough combustion temps and intake/exhaust velocities to burn everything completely and keep deposits from collecting. For those people, conventional oils might indeed be cleaner on average. And I'm sure there are plenty of people in the middle for whom conventionals and synthetics are comparable in terms of deposit formation.

Thoughts?

 

You have some valid points here IamFodi. When I ran my track RX7 (turbo and up to 400 hp, depending on tuning and fuel) I ran on full synthetic Amsoil and premix, just like the pro IMSA RX7 teams were running at that time. We were running at almost constant high rpms on the track so we had the heat profile to burn off all injected Amsoil and the premix completely. We had all the lubrication benefits of using full synthetic and premix with no apparent deficits of buildup.

BTW- IMSA Championship RX7s of the time could run the entire season on the same engine and at seasons' end these engines looked amazingly clean on teardown, while competitors from Nissan, Toyota etc. were going through new engines each race or two. Yes I was often running CAM2 fuel, which was very high octane (available at the track).

Yes Renesis engines aren't exactly the same as 13Bs, but comparison can be valid.

 

While it isn't a perfect relationship, I think we can use flashpoint as a method of comparing between conventional and synthetic oils.

I don't think that the flashpoints differ enough to between conventionals and synthetics (as groups) to support your hypothesis.

More Than You Ever Wanted to Know About Motor Oil

 

I really feel like I must be missing something here. Hopefully we can get some more feedback. But if I'm right, I think this would neatly tie together most of the seemingly contradictory evidence on this topic.

It'd also lead to a pretty straightforward set of recommendations:

- If you’re always driving the car as it’s meant to be driven, you should be running a synthetic.

- If you’re always driving mildly, you might be better off with a conventional, or a semi- or full-synthetic with relatively high volatility and a relatively low flash point

- Mixed hard/mild driving might be tricky. Go with a synthetic and ELIMINATE idling, short trips, and low RPM driving -- or go with a conventional and try not to run it too hard too often.

Tangential: I know a lot of the fully-synthetic rotary oils out there, including Mazda's own, tout ester content. Esters leave fewer deposits when burned, and have some ability to clean up their own mess due to their polarity (credit: Tom NJ on BITOG). Could be another factor.

So yeah. Fingers crossed for some more feedback.

 

None of it matters anyway. Just run what you run an change it often.

 

Good article. Ed Hackett seems like the real deal -- one of the few people still posting on BITOG who are legit experts.

I've seen the argument that synthetics don't have flash points much higher than conventionals, on average. Amsoil uses that argument in its "TSB" on synthetics in rotary engines. Of course, they also tout their higher flash points as indications of their superiority, so... But either way, it's a good point.

In the conversations I've had with lubrication specialists, I've been told that a flash point difference of 20º or 30º C can make a big difference in an engine. I think this is party because a small difference in flash point often corresponds with a big(ger) difference in how quickly and easily a substance burns at temps just above that flash point. Above 1000º or something, yeah, any oil is going to burn and differences are going to be minor. That's more or less what Amsoil said to me on the phone to explain/defend the aforementioned TSB. But that's a different scenario from just sitting in the 500s.

AFAIK, fire point would be a better indicator, but it's not published for most oils. Flash point is best used as a proxy, albeit a highly imperfect one as you suggest.

 

Also, what I described doesn't require a huge margin between synthetics and conventionals in terms of combustibility. If the Renesis is capable of conditions on one end of that margin while some engine oils are on the other end, there's a potential for problems no matter how wide the margin is.

Self-cleaning is important, too. An oil that has a harder time burning might be just fine if it does a good job of cleaning up deposits. But that's harder to assess.

 

This is all wild speculation based on incomplete data. We don't even know how RP arrived at their temperature (specific methodology) to be able to evaluate the quality of their measurements.

 

And RP is overpriced junk anyway.

 

We really nees to bury the notion of synthetic oil. That's not a useful term, it lumps together all types of chemistry that doesn't describe any particular oil. A bit like how the average human is neither male nor female, while obviously the vast majority of humans are one or the other.

The are different base stocks, some which are more prone to leaving deposits, some which are better at dissolving deposits previous oil has left behind. Some burn cleanly, leaving additives as ash. Some have zinc, some have molybdenum, some have none of the above.

When you talk about race synthetics that are meant for short duration, ultra heavy duty, don't go thinking that's a suitable product for a street car. There is more than just deposits to worry about.

Also don't forget that Mazda's recommendations were written in 2004, several API oil spec revisions ago.

 

Correct. Guessing they're going by EGTs -- can't imagine what else they might be using. Don't think they're big or sophisticated enough to do anything more exact.

FWIW RacingBeat seems to roughly agree on the upper limit; they say 1600º F for a Renesis. Add a bit to account for the fact that combustion temps are higher than EGTs, and you're in the ballpark.

Do you have a thought on what a more sensible number might be for the lower limit?

 

I have the dreaded 6 speed auto, and drove it 400 miles at 80 mph to the track.

It had some power loss even though BHR ign was put on 10k miles before.

First new plugs were installed at 32k miles, it didn't look like many deposits, and it was loose wire that was power loss anyway.

I've run Mobil 1 0w40 for the last 25k miles and 8-12 oz of Mobil 2T for every 12 gal. fillup.

I think the PIB type Synthetic premix helps the whole deposit control since it's designed to control deposits in 2 stroke.

There are even lots of tests for exhaust port plugging of various premixes on the net.

 

We can get catalyst temperature from the rear O2s. Is that something the front O2s can do?

I don't know if it would be useful because EGTs don't necessarily tell us the temperature gradient for material closest to the water jacket. The combustion chamber won't be a single homogeneous temperature. There are going to be parts that should easily get hot enough to burn off any reasonable oil and parts that may not burn off any.

The other consideration is that the small amount of oil injected may not be a significant contributor. Healthy piston engines will also get carbon build-up in their combustion chambers without any oil injection.

 

Not that is matters, but I consistently measure average EGTs at the rear O2 sensor around 1750F on the track. Surprisingly, street temps are not that much lower. I have graphs around here somewhere...

You should all line up to buy my new book: How I Learned to Stop Worrying and Use Whatever Oil and Change It Every 3,000 Miles (Filter Every 6,000 Miles).

 

To be fair... If I'm right, you guys are exactly the kinds of people for whom it really wouldn't matter what oil you use. (assuming you mean by that what I think you mean)

If I'm wrong, then either Mazda straight-up lied in that TSB, or there's some kind of mystery ingredient/formulation that may or may not be part of any given synthetic oil and kills Wankel engines regardless of usage. Neither seems very likely.

 

We will never know.

 

All my engine dyno sheets typically indicate around 1650*F for egt @ at max rpm and 13.1 AFR give or take a few. A MoTeC ecu was used on the dyno runs, not a factory pcm. I wouldn’t really trust the OBD2 reading for being as accurate as a true egt setup, which I’ll have on the new header currently in progress. Of course there are orher factors too like tight seals likely running cooler than loose seals that bypass egr back into the intake side, fuel type/quality, actual tune fuel/timing quality, whether the factory pcm is pulling some bs software alteration that was programmed in on a later update to keep power levels in check, and so on

 

WTBRotary’s Daryl Drummond built engine dyno

 

Nice. Thanks, Team.

Any idea what your EGTs were just sitting at idle on a warm engine?

 

So my local Mazda service dept says that they have seen synthetic oil gumming up the oil ports causing premature failure of the lubrication system. Your thoughts? I'm currently using Royal Purple 5W-20 in my series 2 so I am concerned.

 

The avg dealership doesn’t have a clue, but you should stop smoking the purple oil crack regardless.

 

They may have seen clogged oil injectors on a car that used synth, but that doesn't imply synth caused it. Somebody running regular oil and running past the change interval is far more likely to accumulate deposits.


They're also not allowed to open the engines, so the scope of they could have seen is limited.

 

^ This.

Technically any oil could cause this under the right conditions. But I'd imagine the engine would tend to fail for other reasons first, especially with a synthetic.

As for Royal Purple: It's not awful, so don't let anyone make you **** bricks for using it. But it's not great either and most of the claims behind it are worthless, so it's true that you can do much better.

 

Only some S2s still have any Mazda engine warranty left, so use whatever oil you like. Just change it regularly.

(BTW, my 2008 RX8's owner's manual does state to not use synthetic oil ). And there have been a few cases of S1's denied warranty engine replacement by dealers because of documented synthetic oil use. Some of those reported denials were eventually overturned by appeal to MNAO.

 

That's why (under warranty) I use WallyWorld's SuperTech 5w20 and change it very often.