Oil consumption - Synth vs. Dino

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Feb 5, 2003
Columbia, SC
Hey everyone. First time poster. I've been looking through some of the threads and have found them very informative.

Here's my deal. I have a '99 Miata with 40000 that has been running Mobil 1 10w-30 for 38000 of those. I have noticed for the last couple of changes that I've been down about a quart. Since the last change about 1500 miles ago, I was down about 1/2 quart. I change oil religiously at 3000 miles.

I've heard conflicting things from different people regarding syntheitc vs. dino oil consumption. The Mazda mechanic went so far as to say that in the case of the Miata engine anyway (4 cyl, 1.8, redline at 7K RPM), synthetic should not be used and that I should be using dino oil only.

My questions are: Is there any relation between using synthetic and seeing the consumption of 1 quart in 3000 miles? Would dino fare any better? 1 quart in 3000 miles seems a bit excessive in a car with 40000 miles on it.

For what it's worth: No leaks/seepage found anywhere, tailpipes stay reasonably clean, no smoke from the tailpipes (except a white vapor when car has sat for 30 minutes or so which goes away after driving). Also, plugs are completely clean, and the oil looks normal.

I'd appreciate your feedback. Thanks!
Are you saying that it never used to use oil until the last couple of oil changes? That's weird...maybe PCV is plugged up or something else is going on. Air filters changed? Vacuum leak? If you used M-1 10-30 ev. 3k...your engine should be spankin' inside.
I've only noticed it for the last couple of changes.

PCV was replaced before the last change, at since then 1500 miles into it, I'm down 1/2 quart.

Filter is clean and no leakes around filter. Strange indeed, unless that's in the normal consumption rate.
Why not try something just a little bit thicker? You could try another 10w30 such as Redline or Royal Purple or Amsoil which are thicker at 100c than Mobil 1.
I've been following your tread on miata.net and a new pcv valve could be the problem if it didn't open and close at the proper times. Maybe Dr T has something. Why a new pcv at only 40,000 miles?

Originally posted by Dr. T:
Maybe PCV is faulty (even though new)

I agree. Generally syn use will be less. You could add a quart of the 15W-50. It cut my oil use in one vehicle about to about 2/3 of the origional usage.
Well, the oil use appears to coincide with the PCV change, all of a sudden. Follow that lead to ground before you start changing an oiling routine that has served you well.
I'm sorry, I should have been more specific with the PCV valve. After the first time I discovered I was a quart low (last oil change), I inspected my PCV valve, and found it to be all "gunked up". At the same time, I was getting leakage out of my oil cap. At that time, I replaced the PCV valve and the oil cap (a nice new shiny one...mmmm....shiny stuff). The leakage stopped at the oil cap, but apparently the oil loss continued. 1500 miles into this cycle, I was down about half a quart.

I could try a new PCV valve. I'd rather keep on using the same oil, as entirely too many people use it with excellent results.

I could try another PCV valve to see if that helps any.
Hi Mr. Roper!

M1 10w30 gives me the same trouble with my turbocharged '94. I think it's classic M1 in that I'm fine for about 3k miles and then it starts to go away. Others have recounted similar things and there's been some suggestion that it's because M1 is on the thin end of the scale and tends to thin a bit more in use, allowing for increased consumption. Consider a thicker oil (either another brand or a thicker grade of M1). FWIW, in the summer I run a 50/50 mix of 10w30/15w50; seems to help a bit.

Thanks, everyone.

Here's another thought. I'm on my second PCV valve. The first one I replaced at 30000 as part of the routine 30000 maintenance. I noticed that on my friend's '95 Miata, the PCV valve fits rather snugly into the grommet. It takes a little exertion to get it out. My PCV valve comes out very easily. It's not like it's rolling around in there. It just pops out with no effort at all. Could it be that the grommet is a little worn and the valve is not seating properly?

As far as mixing 10-30 and 15-50 - Are there any downsides to that?

Porterdog - I'm not even making it to 3000 miles. At 3000, I'm down a quart, and at 1500, I'm down half a quart.

[ February 07, 2003, 08:09 AM: Message edited by: Mr. Roper ]

Originally posted by Mr. Roper:

As far as mixing 10-30 and 15-50 - Are there any downsides to that?

There doesn't appear to be. Al has an oil report on here where he mixed in 15w50 with 10w30 Mobil 1 and it showed nothing wrong. I know a few LS1 guys who do this too. I sent Mobil 1 an email this morning just to see what they say about mixing the two.

Originally posted by Mr. Roper:
Thanks, Patman. Please let us know what they said.

Still no response yet, but then again I sent it just a few hours ago!

I've found Mobil 1 responds quickly to their email, however they usually don't give very detailed info. I also asked them for the HTHS data.
I'd say to just try adding 1 qt of the 15w50 into the mix at first, and see how that goes. As Al said, it worked very well for him at reducing his consumption.
One last thought on your Miata. Could the throttle body and intake be in need of a good cleaning? I have read on this board that this can increase oil consumption. I know the Miata is bad for this also. Just a thought. My guess is this will need cleaning about every 50,000 miles from the post on miata.net.
I got the email back from Mobil 1 just now, here is what they said about mixing 15w50 with 10w30:


Mobil 1 10W-30 and Mobil 1 15W-50 are fully compatible with each other.
There would be no adverse use effects. You would lose the fuel economy
credential the 10W-30 has with the mixture though.

Thanks Patman.

So would you recommend a 50/50 mix? Or maybe 3 Q 10w-30 and 1 Q 15w-50? What do you suggest?

I'm in southeast USA, so it does not get terribly cold here.

[ February 07, 2003, 01:01 PM: Message edited by: Mr. Roper ]
I'm suprised no one mentioned the fact that M-1 is gunking up a PCV valve at 3k intervals. Maybe it is only because of the poor seal though. Am I the only one starting to lose a bit of faith in Mobil 1?
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