I've been logging oil pressure and temperature in my RX7 for a few years now, using the replacement filter pedestal from Banzai Racing and a stainless 0-150psi sensor from AEM. This past winter I switched to 10w40 generic brand oil (NAPA auto parts) with hopes of slightly better manners when ambient temps were freezing or near freezing. It didn't seem to make much difference for cold engine behavior, but when the weather got warmer the ECU datalogs showed oil pressure was not maintained at high RPM. This log screenshot shows the normal 'data vs time' traces on the left side, Oil Pressure is the orange trace in the second plot. The top-right side of the screenshot shows an XY scatter plot of oil pressure vs engine speed.


10w40 oil, note pressure drop at 6500 RPM



The oil and filter had not seen many miles or hours of running, so I swapped the oil to the usual 20w50 Valvoline without changing the filter. Even at hotter oil temperature, pressure looked good again. I was surprised to see this much difference from just changing the oil brand and viscosity.


20w50 oil, pressure looks good even at higher temperature


I've put more miles on the car with the 20w50 oil since that datalog, and pressure still looks good so I don't think this was a fuel dilution problem. Or if fuel dilution is a factor, the 20w50 handles it better because logs have not shown pressure problems in the previous few years running the same 20w50 oil. The car has a stock engine, no oil mods, base model RX7 with the factory single oil cooler.

 

Awesome data, thanks for sharing.

Is your engine stock, oiling wise, other than the pedestal and sensor?

 

Very nice.

 

Well, I'll be dipped in ****, Scotty!! Interesting and COMPLETELY unexpected. Appreciate the insightful post.

 

Some what related. Oil testing on a 13B Rew.

 

That youtube video helped encourage me to make this thread. I saw it after switching my car back to 20w50 oil and confirming the pressure was good, and realized that I have some info that might help others too. The factory oil pressure sensor and wiring harness is old enough that I might not trust mine anymore, if I had seen the oil pressure gauge on the dash dip low I might have just ignored it. Plus I'm not watching that while I'm driving at full throttle anyway.

But yes, everything about my car's oil system is stock besides the Banzai filter pedestal for adding sensors. Stock unopened engine, it's from a ~1997 JDM car and I've added about 25000 miles since I installed it in 2010. Stock oil lines, single stock oil cooler (base model). I'm not sure if a dual cooler setup would be better due to lower temperature or worse due to the additional pressure drop of the second cooler. I bought a big aftermarket oil cooler and thermostat a couple years ago, but haven't gotten around to mounting and installing everything yet. It will probably be a while before I get that done and have additional info to share, it's not a big priority for me.

 

While annoying to see the dip, mazda specifies the REW oil pressure to be satisfactory if 50psi or above at 3000rpm. This is all it says in the manual. Presumably, that could be extended to minimum 50psi at any rpm above 3000? Not sure if that's a fair extrapolation. So while flat lines are better than wavy ones in this case, wouldn't that make the dip technically acceptable so long as it's maintained above the minimum?

 

That's a very good point that none of the dips were below Mazda's min allowed 50psi, so it might not be the end of the world to drive around with that oil in the car. But for me it was really easy to change back to the previous brand and viscosity, rather than wonder what would happen at the next track day or autocross event. I haven't done an HPDE / track day since installing the sensors, but I have seen occasional dips during autocross days even when running 20w50. Without steering angle or lateral acceleration g's it's hard to see the cause from datalogs alone; I'll try to catch some runs on video to get a feel for what situations causes the pressure dips during autocross runs.

 

My concern with 20-50 came around when I tore down my first 20-50 motor. The oil felt and looked like actual syrup...... for pancakes. It was so thick and slimy and just so weird that I questioned what it even was.

That youtube video confirmed my suspicion about using heavier weight for the fuel dilution resistance alone but after playing in Aunt Jemima, I decided it wasn't worth it. Cold syrup until fully warmed up just didn't seem cool to me especially since 100% of my driving is on the street and I don't use e85. The potential benefits of 20-50 just didn't seem to translate...... for me.

Been using 10-40 semi synthetic for a while and recently met an FD owner who has been using 10-30 conventional since he bought the car in 94. So long as you stay up on your oil changes, I think for street cars it doesn't matter as much we may think.

Other than the excess fuel dilution and extreme heat in some set ups, I feel like theres no reason to not use a lighter weight. The oil chart in the manual shows that 10-30 is the recommended oil anyway. Of course, it's a use case basis but I think defaulting to 20-50 isn't a good practice overall. We run the majority of our customers on 10-40 with a few exceptions for the reasons stated or if they simply have a preference.

Without doing bearing comparisons over some period of time, I think its hard to say what the extended effects of the individual weights are. Of course logging temp and pressure as was done here gives an idea of something but bearing wear I think is the ultimate deciding factor for oil weight.