Best oil for high mileage Miata

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What would be a good oil choice for a 1990 Miata with the 1.6 DOHC 4 cylinder engine, and over 226,000 miles on the clock? I have done the 2 bottle Auto-Rx clean & rinse process, and I am currently running Castrol GTX 10W40 dino. I am burning about 2 quarts per 3000 miles. The engine runs great otherwise. Should I run a thicker dino oil? Synthetic? Thanks.
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2 qt/3000 miles is not even considered a problem on new cars by manufacturer's. Oil consumption is normal especially in a 14 year old 226k mile engine. I would just recommend a High Mileage oil that is the correct spec for motor. No reason to run synthetic on a motor that old especially if it has not seen that. And its expensive to top up with synthetic vs conventional.

Lastly I notice you are from New England 10W may even be too thick dependant where you are. Don't do anything over 10W if you start the car in colds temps.
 
Go to Wal*Mart and get their SuperTech $0.82 oil and add a quart every 1,500 miles and change at maybe 8,000 miles. You will have so much top off oil that you are running "new" stuff all the time.
 
Since you've already cleaned the engine up by using Auto-Rx, I'd suggest going for a 15w-40 to see if that helps, namely Chevron or Penzoil's products due to the lower NOACK volatility rating. Perhaps even the Pennzoil or GTX HM 10w-40 or 20w-50 products might be worth a shot.

If that still doesn't work, there's dino GTX 20w-50. I wouldn't use synthetic since you'll be consuming some expensive oil. Keep the money in your wallet.
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Let us know what you decide on and what your results are.
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If the car is garaged you might could try Castrol , Quaker State or Phillips 5w-50 .

You being a long time owner you probably know those really hold just 3.5 quarts with filter change even though many install 4 quarts overfilling them so if it's the rings overfilling will only increase the oil burning from the poor ring seal and increased crankcase pressure .

That 10w-40 GTX will be a 30wt pronto in most engines so it's not as thick as you might need .

The 10w-40 Mobil HM Syn Blend might help too and less money than the 5w-50's above that would only allow in part to be used because of the 5w yet be thick enough to help ring seal . A poor Miata oil in cold weather though .

Miata's make ugly noises when the oils are too thick , even in warmer climates and the Miata oil pump problems we read about are owner oil choice induced .
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IMO

Could it be possible your Catalytic Converter is clogged or the muffler is internaly collapsed ?
Does it spin to redline with ease like when newer ?

Have you performed a compression test ?
 
The muffler is fairly new. The converter is original, but the car passed recent emissions test just fine. The engine goes to redline wth no problem. Haven't done a compression test in years.
 
Would Mobil 1 15W50 be a better choice than Castrol GTX 20W50, or am I just going totally insane?
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I know it costs more, I just want the best protection for my engine, which still sees frequent trips to its 7,200 rpm redline.
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Thanks.
 
Mobil 1 15W-50 has worked for me in the past. I have a '90 Miata with 238,000 miles. It consumes no oil between 5000 mile oil changes. It has had M1 15W-50 from the time it was new (after a 2000 mile break-in). The last belt change was at 180,000 miles, and wear patterns were just visable on the cam lobes. I do not have a UOA, as I am a new member to this forum. Unforunatly, the Miata is being sold to a co-worker, as I just purchased a new RX-8. My other Miata, a '97, has also had M1 15W-50 in it from the time it was new. I will be able to post a UOA on it in a while, so the forum will have hard numbers to work with. Bear in mind, that I live in Houston, Texas, and have to contend with 100+ F days. Your requirement in the Northeast may be radically different than mine! Enjoy the Miata, and keep us posted!
 
I don't know if this helps but with Castrol 20w-50 in my toyota at 260,000 my consumption is 1 quart every 1000 miles. If I go to a 10w-30 my consumption goes to 1 quart in 300 miles. The thickness of the oil makes a great deal of difference in an old engine. I would go with a 20w-50 or Mobil 1 15w-50 and I think you would be OK even in New England with the Mobil 1.
 
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