Castrol GTX 5w-20 seems to be the non-synthetic 5w-20 of choice for Mazda and most Mazda dealerships, which means it's the closest thing to an official specific oil recommendation for the RX-8 (and some other cars).

The publicly available info on this oil provides no evidence that it's anything special, so it's nice to see testing that at least confirms it has decent specs. PQIA does this testing to verify that oils being sold to the public are within spec and good for use.

Results: Castrol GTX SAE 5W-20 API SN/ILSAC GF-5 Motor Oil

Few standouts:
- Viscosity @ 100º C is way on the high side for an xw-20.
- Levels of zinc/phosphorus (proxies for certain anti-wear additives) are not amazingly high, but close to the limits for API SN; should be plenty assuming they're using remotely modern additive chemistries.
- Decent slug of calcium, which is usually related to detergents and corrosion inhibitors.
- Moly and boron are at decent levels.
- Volatility isn't great but well within the limit.
- TBN is solid, especially for the short drain intervals most people here (hopefully) run.

In other words, it's fine. There aren't a lot of oils with better numbers than these. It's not even clear what "better" would mean anyway, since more isn't always better.

Or at least, these numbers are fine. Anything that's better for a Renesis would probably owe its advantage to properties that aren't tested here (or in almost any oil analysis).

FWIW.

 

And the best thing you can do is run at Least 5-30 😎

 

looks good for a reciprocating engine, not so sure for rotor seals ...

 

Why's that?

 

I have some pics here somewhere, but we have a local member who is a Mazda tech who worked at two local Mazda dealers and I took pictures of the oil tanks and neither used Castrol. One used Mobil 1000 (or whatever it's called) and the other used some sort of Shell and it was actually a 5W-30 IIRC.

GTX is decent stuff. But this was my buddies Mazda reman after 30,000 miles. It Ran Castrol GTX 5W-20, meticulous oil change intervals, etc.

 

Reciprocating don't generally burn oil, certainly not intentionally like a rotary engine. How do you know what the impact of that stuff burning on and building up on seals is? I'm not sure myself, I just understand enough to ask questions rather than blindly drink the mad engine oil scientist koolaid ...

then you street guys with cat converters need to consider what that means for cat life too ...


.

 

Every time the viscosity thing comes up, this big asterisk appears in my head...

Here's a presentation from Shell that I had seen a while back (archived because the original site is down): https://web.archive.org/web/20050309...itives2001.pdf

Check out slide 18 -- minimum oil film thickness (MOFT) numbers under the top piston ring in a test engine. The 5w-20 they tested showed higher MOFTs than a 5w-40 and a 15w-40.

This doesn't say anything definitive about any other part of any other engine; it just means a thinner oil doesn't necessarily mean thinner films, as counter-intuitive as that might seem.

AFAIK this is especially important when comparing "conventional" oils. All xw-20s need at least some synthetic component or they'd be too volatile, whereas a 5w-30 or 10w-30 can be 100% conventional. Because those synthetic components retain viscosity better, there might be a bunch of high temp/high shear regions in the engine where a "conventional" 5w-20 might lay down thicker films than a conventional xw-30.

 

The deposit/buildup and cat converter issues are why it's not necessarily a good idea to use something with more elemental additives than this. It's also addressed somewhat by the API Starburst specs; oils that don't carry it might be the same or better, but it's hard to know.

Using a thicker oil might help keep those additives in the oil better, all else equal. Hard to tell whether all else is equal, though.

A synthetic would be even better in that respect (and others), though I'm sure we're all familiar with the debate there.

All of this is why I'm hesitant to stray from Mazda's recommendations...

 

Interesting. All I've heard from the dealerships around here is Castrol Castrol Castrol.


That's probably more likely a mechanical issue, no? AFAIK very few engine failures these days are clearly attributable to engine oil choice unless it's WAY out of spec or something.

 

You should got to the dealer and investigate, I highly doubt any of them use Castrol GTX as it is more of a store brand that is sold.

As for the failure, could have been. I wouldn't say very few failures are not attributed to oil choice. I have seen quite a few locally that were IMO, but really I don't know ****. And I have seen a few that were poorly maintained using 5W-20 for way longer intervals than they should have run it.

But without actual scientific testing we will never really know.

 

thank you for this highlight...i read something like this before where w-20 oil can actually protect better than the w-30...

 

So is the Mazda Genuine Oil better then the Castrol GTX?

 

This bearing pic implies that 5w20 oil isn't thick enough. After being an Exxon industrial oil marketer with lots of oil analysis, I went straight to Mobil 1 0w40, as did 9krpmrx8, and some others. bearings should easy to lube, it's the hot and variable speed cam lobes that used to be trouble.

Another advantage of new SN synthetic oils is the zddp additives are mild, since no hard to lube valve train and cams. Therefore, it's easy on cat converter, and should make less deposits. Lots of the dino 5w20, 5w30, 10w40 used STP type viscosity improvers that made lots of hard carbon.

Most of these synthetics like 0w40 or 0w30 start as natural gas, and are built up.

I'm running Mobil 10w40 in everything including BMW 2.0 turbo.

Read up on BITOG.com for eye opening used oil analysis of many vehicles.

I see no need to run diesel oils like Shell 5T 5w40 since all those extra additives for diesel soot might leave more deposits, but every one gonna do what they're gonna do.

 

Nobody knows and it's a moot point anyway.

Pick any decent brand oil and run it at reasonable intervals.

 

Interesting. How do you rule out mechanical defect, assembly errors, poor oil system function, bad driving habits (e.g. too much RPM when cold), etc. etc.?


Mobil 1 0w-40 is high on my list for after the warranty is up.


Indeed.

Was thinking low-SAPS oils would be worth a look for similar reasons. Most of those are synthetic though AFAIK, so this is another thing I'm postponing until after the warranty is up.

Thoughts on the relevance of HTO-06 approval? Do you know of any other OE specs that try to get at high temp deposit formation in used engine oil?


Are they all GTL these days? I thought they were still various mixes of PAOs, esters of various kinds, and Group III basestocks (GTL or otherwise).


I'll just say I'm quite familiar with BITOG, and fairly familiar with the benefits and shortcomings of UOAs. :]

Not a hell of a lot on RX-8s there, and the good info tends to get utterly buried by the blind leading the blind. Not much else on the Internet, though...

 

You are way behind the curve on this.

I had a 73 RX3 and now 07 RX8, and the same parts are **** on both: ignition, shocks, and fuel pump.

Every car company touts their OEM oil as perfect even if they don't know what's in barrel at moment.

That slide show is hard for me an oil marketer to obsorb, but we do know that when metal gets worn that badly, thicker is needed.

Mobil 1 0w30 isn't even as good as 0w40.

Some cars thrive on 0w20 or 5w20 (Toyota and Honda), but don't drive **** outta car with it in.

You and the slide show talked about boundary lubrication, and they have a test for that: High Temp/High Shear of which 3.5 is considered minimum to stop wear.

Those oils as low weight as 0w30 or 0w20 are considered "Energy Saving" not engine saving.

Go look up HTHS for GTX 5w20, and notice it's not minimum of 3.5.

 

Technically yes, if there's wear, then by definition the MOFT wasn't enough. But that doesn't rule out some other problem with the engine that, if corrected, would have prevented that wear even with the same MOFT.


Not sure what you mean by this.

HTHS is a viscosity measurement. Boundary lubrication is where the oil film has been penetrated, so viscosity isn't as important as additives. When you say HTHS is "a test for" boundary lubrication, what do you mean?

"Stopping wear" isn't actually possible in an absolute sense, but I'm assuming you didn't mean that literally. Either way, I thought viscosity requirements, including HTHS, depended on bearing clearances, metallurgy, finishing, expected oil temps, and the like. I wasn't aware of any such thing as one universal minimum for all engines regardless of design or construction.

But as I'm so far behind the curve, maybe you can enlighten me.

 

Yeah, my explanation didn't hit the mark; I mean you seem to over think it.

The thinnest oil that works is close to the best vis.

The bearing in photo is under pressure, and should be easiest to lube, but not even close, so needs thicker.

Boundary lubrication would be where no film to slide on like starting up, but AW additives like zddp react with metal as final barrier.

I've seen very good results from Castrol Edge 0w40 and Mobil 1 0w40.

Some engines are speced for 0w20, but if I'm driving hard in nearly any engine, I'm gonna run approx 0w40 to SLOW WEAR NOT SAVE GAS.

No need to wait for warranty up while you damage bearings; Put the 0w40 you want in. Service manager doesn't even know whats in his tank, and isn't going to test your oil.

 

Yeah, there definitely seems to be some kind of disconnect here. A lot of what you're saying makes sense, but points keep shifting and I'm still not sure how much of it relates more than indirectly to what I posted.