Seeking advice on how to handle partial oil changes

[shocar]

Hello,

I have been lurking here for a while and have come up with a question. To show how astute I have been in reading advice given on this web site, I will pose my answers to the question also.

I have a new Mazda RX-8 which I really like and want to keep running forever. I am also just as much of an oil "enthusiast" as practically anyone on this board.

The RX-8 hold 7.1 qts of oil. When draining the oil only 3.8 qts drains out. The remaining 3.3 qts stays in the oil coolers and rotor bearing/gear cavities. How should this affect my oil change plan? Does the recommended use of 5W20 oil affect the plan? Does the recommended not-to-use synthetic oil affect the plan?

My current plan is to go ahead and change the oil beteween 2000 and 3000 miles.

Some anticipated answers would be to have oil analysis done after a couple of 5000 mile changes to see how stable things are. Another comment would be to start using Lube Control. This would help handle the contaminated old oil, correct? Any recommendations on the oil? I cannot say I am happy about using 5w20 oil, but have read that 5W30 is practically the same after 3000 miles running. I think there is a little tendency for the rotary engine to dilute the oil with some fuel. Again, this is a subject for oil analysis. Thanks for any advice.

 

[Eddie]

I would first follow the recomendations found in the owners manual as to oil viscosity and change interval. After a good break-in I would change oil/filter using the savere service interval for two changes and have the last changed analysised. If that looks very good then I would run two oil change interval using the "normal oil change interval and have the last change analysed by the same laboratory. I would NOT try to second guess the factory Engineers on the type of oil to use ie synethic or dino. I would also not add any make up oil unless absolutly necessary as it will foul up the test comarisons.

 

[loudist]

Is there a way to send pressureized new oil backwards through the oil filter fitting thereby exchangeing the oil in the cooler and bearings?

 

[1sttruck]

If you're only getting 3.8 qts of new oil in on a 7.1 qt gump, you're leaving 1-(3.8/7.1) in, which is 46.5%, and considering subsequent changes you'e leaving [1-(3.8/7.1)]^n behind, n being the nth change:
1 46.5%
2 21.6%
3 10.0%
4 4.7%
5 2.2%
6 1.0%

A 'normal' chnage might be 1 qt left in 6, so maybe you want to use an oil that is good to the 3rd change interval; if you do 3k changes then an oil good to 9k seems like it would get you a bit better than what most engines see. I think that's right, but if it's not you can count on this group to point it out :^)

 

[unDummy]

Don't worry about partial oil changes. Rotary owners have been doing this for years, most not even knowing it.
Change your oil frequently. This will keep the metering oil pump happy and help prevent early failure(clogging) of the oil injectors.
Your 2-3k OCI is perfect.

Oil weight isn't an issue. The engine has 2 oil coolers and I'm assuming oil thermostats.
With a 7.1 qt capacity, the oil temp and viscosity is kept in check. The 5w20 will provide the best performance.

There is no easy way of flushing the oil out of the engine. Don't bother trying.

The UOAs on 5w20 oil are great. Brand choice is yours to make. Most perform well.

I also think that Greddy has a larger oil pan kit. This will allow you to change out a higher percentage of the oil.

The owners manual is just a recommendation and as such is good reading, but not the Bible. If you want to keep the car a long time, avoid its recommendations and maintain ALL fluids/filters more frequently.

 

[AEHaas]

The normal oil change in my 575 Maranello only drops 12 of 15 - 16 qts. But if I take down the lines from the oil tank to the engine and the oil cooler then I get a full change. It is just a few extra steps, add 15 minutes.

aehaas

 

[shocar]

"Is there a way to send pressureized new oil backwards through the oil filter fitting thereby exchangeing the oil in the cooler and bearings?"

Well the Valvoline Instant Oil Change shop nearby has a Bilstein oil flush unit that could be used for complete changes. But I don't want to go to the expense. Plus I would have to bring my own oil to be used.

 

[ebaker]

quote:

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But if I take down the lines from the oil tank to the engine and the oil cooler then I get a full change.

---

The additional wear you may get while the oil pump is refilling the empty oil coolers would not be worth it. When you restart, the time at near zero oil pressure is increased if you drain the coolers. After you refill the oil tank you could crank it with the fuel pump relay pulled until you get oil pressure. Thinking of a $100K motor rattling while you wait for the pump to reprime the system makes me cringe.

 

[Audi Junkie]

Just do a flush with a few extra quarts. Lord knows, I would.

 

[shocar]

Thanks for the suggestions everyone. I had thought about the dilution ratios over successive oil changes as well. This seems like kind of an interesting problem. I don't mind changing the oil more frequently. Mazda has once again reiterated their stand against using synthetic oil in the rotary engine. I would have prefered to go with a nice Mobil1 or even Amsoil. I think that Mazda will have to approve synthetics eventually. The latest oil technologies allow the use of some synthetic components in regular dino oil do they not? The manufacturer would not have to claim it is synthetic oil.

 

[MikeR]

The rotary burns a bit of oil by design as well, right? Seems like I read somewhere about a quart per 1000-1500 miles. I'd feel good about changing dino oil every 3k.

Heck, what am I saying? There's very little expense with dino, and that's a very nice new car. I'd drain and refill twice (running it for a while between) at 3k mile intervals. Now as to whether you'd change the filter on the first or second drain...

good luck!
mr

 

[k1xv]

I don't think this is a problem so long as you conform with Mazda's oil change frequency recommendations.

It seems to me that the purpose of an oil change is to get contamination/worn oil out and fresh oil and fresh additives in. So, for example, if only half the old oil came out and you replenished it with fresh oil, immediately after the oil change you would have oil in the system equivalent to "half depleted", but still effective, oil.

Mazda has been working with these rotaries for over 30 years. I am sure they set the oil change frequency recommendation to get some worn oil out and fresh oil in to replace it before the oil loses its effectiveness.

 

[1sttruck]

If you're the typical US driver who drives 55mph in the #1 lane, use whatever oil your maker recommends. If you know that oil recommendations in the US are designed for mileage improvements, that heavier oils are usually recommended elsewhere, and you use the performance in your vehicel once in awhile, find out what oils are being used in other parts of the world and then make your selection. If some light oil is being used in Germany then it should work fine in most situations over here.