Hey guys! I picked up a 91 (NA) Miata with 33k miles on it. I'm planning my first oil change. The dealership did the changes for the prior owner for the last ten years, and they said they used 5W-30 full synthetic.

On the Miata forum, those cats seem to lean toward 5W or even 0W. The idea being better startup lubrication -- faster flow I think is the theory. I want to buy this argument, but then I start to think, as I'm warming the car, for the first five minutes of driving, wouldn't that oil be a little thinner than desired until it gets up to operating temperature. Because I'm probably starting the car at like 50F - 70 degrees, most often (early morning, late evening drives). So wouldn't I get oil on the parts faster at startup, but the oil is a bit "watery" for lack of a better term for a while, the parts are less protected?

They seem to indicate that 0W-30 covers all of 10W-30 and more. So there's NO reason to use 10W-30 nowadays. If when the car was made, they just didn't have these grades, that would make sense. But the manual seems to show that they are aware of 5W-30, and they do not seem to recommend it for above 32F.

I overthink things, it's what I do...



Obligatory sunny day car pic (with cat photo-bomb), and Jackpoint jack stands FTW! I am in the middle of doing the gear oil and differential. Stuck waiting for slow shipping crush washers!

Oil viscosity was a recent topic in this forum.

Please remember that oil viscosity is a function of temperature. It is logarithmic. The viscosity will be what it is, for any specific formulation.

For your starting temperature of 50-70 F the lowest to highest relative ranking of viscosity is obviously 0, 5, 10. However the difference in the three viscosity will be much less at 70 than at zero.

If the dealership oil choice has worked for ten years, why change?

The owners manual chart makes no sense. Please read the other thread and/or review the SAE J300 viscosity chart.

I’d lean synthetic 5w-30 or so. Did you say what State you’re mostly driving in?

The other thing you mention is 2-3 minutes of warmup after start.

I loved my Miata-best convertible for the price. Make sure you unzip the rear window b4 unclamping the convertible top. Drive and enjoy.

quote:

Originally posted by sourdough44:
I’d lean synthetic 5w-30 or so. Did you say what State you’re mostly driving in?

The other thing you mention is 2-3 minutes of warmup after start.

I'm in PA

I put 250,000 miles on my '91. It would see redline every time I took it out and I typically changed up at 5,000 RPM. It was a great little engine, requiring very little maintenance. All I did was oil, plugs, filters and timing belts. I ran 10W-30 synthetic, 5W-30 in the winter. Drove it year round, with snow tires of course.

quote:

Originally posted by 4MUL8R:
If the dealership oil choice has worked for ten years, why change?

Because I have absolutely no faith that anyone there knows or cares what's best for a 34 year old car. They have kids changing oil, and for the vast majority of the work they do, it's all on modern stuff that either likes the thin stuff, or at least, is specced the thin stuff due to the EPA requirements.

I'll look for the other thread as well. thanks

Those are some very nifty jack stands. And a sweet little convertible.

The other side of the argument is as the engine wears, tolerances get larger, you should use a thicker oil. And that factory recommendations are really for when the engine is new and they only care about meeting certain test numbers at that time.


The problem with this argument is, both sides make sense. Both sides are kind of true. So people pick a side and fight it out on the internet.

r0gue-
Off topic, but I have two of those factory Miata wheels. May have center caps as well. We are moving soon, and I’d hate to trash them.
Free for the taking.
Email in profile.

quote:

I want to buy this argument, but then I start to think, as I'm warming the car, for the first five minutes of driving, wouldn't that oil be a little thinner than desired until it gets up to operating temperature.

No, while a 0w30 will be thinner than a 10w30 as they warm up, it will still be thicker while warming than both oils at 212 degrees where they’ll be the same thickness.

In other words, the 0w30 will move through the engine easier than the 10w30 until 212 degrees where they will be the same and neither oil will be thinner than they are at 212 degrees at any temperature below 212 degrees.

I don’t know about your car, the difference between a 5w40 and 15w40 is noticeable. The engine is sluggish for a longer period of time when started cold with the 15w40 vs the 5w40. The colder it gets, the more noticeable it is. At 80 degrees in Florida, it takes a minute longer for the sluggishness to go away. But at 40 degrees in Northern Michigan, it takes a good 10 minutes.

The dead cat in the driveway pic is a nice touch!

I would not go against Mazda's recommendations, no need to "exceed" the specs that they deemed appropriate.

quote:

Originally posted by architect:
cat in the driveway

Looks like someone has a feline supervisor!

RE: oil, I'd go with what the manual recommends. In the pic, it seems the factory engineers were well aware of 5W30, and didn't recommend it unless the outside temps weren't going to get above freezing between oil changes.

The only time 10W-30 wasn't recommended was if the temperatures were going to stay below zero F. I only lived in PA for a year, but that doesn't sound like summer to me.

Look into HTHS - High Temperature High Shear, and select oil on this specification less so than viscosity.

https://bobistheoilguy.com/for.../what-is-hths.90798/

In Redline oils, 0W30 HTHS is 3.4, 5W30 is 3.7, 10W30 is 4.4. I suspect the factory spec of 10W30 is to ensure a higher HTHS.

I suspect Redline 5W30 might have a higher HTHS than just about any 33 year old 10W30, and would flow better cold also. For your car, I'd rather give up a bit of cold flow with 0W than give up HTHS. Usually it is thought that the cold flow advantage of 0W doesn't come into play until below 0F I think.

I have had 3 Miatas, I would do what the factory calls for, I would probably stay away from the 0wt stuff. Those motors are damn near indestructible, out first two Miatas(92 NA, 00 NB) saw lots of auto cross action and both were well north of 100K when I got rid of them. The only issue they have is the lifters getting noisy with age and taking a while to pump up(my understanding is that’s caused by sludge build up that slows the lifter pressurizing) only the NA 1.6 did that it had over 200k on it. The only failures we have were coils(I kept 2 in the trunk), 1 clutch (NB) and 1 rear differential (NA).

The chart shows Mazda preferred 10W-30 for normal conditions, essentially for 0F to over 100F. 5W-30 only for winter below freezing.

This was based on conventional oil and back then viscosity enhancers would start breaking down almost immediately, the more VI they used the quicker the oil reverted to its base viscosity of 5W for 5W-30 or 10W for 10W-30. So not only does 5W-30 gradually drop down to straight 5W, it does so in fewer miles because of the 6 x load of enhancers to make it act like 30W when hot. 10W-30 starts as a 10W oil with only 3 x load of enhancers needed to make it behave like 30W at operating temperature which lets it maintain its viscosity longer and still holding 10W when completely depleted.

So if you use conventional oil I'd use 10W-30, and conventional oil might be appropriate if you don't plan on driving it a lot as you'll be changing oil because of time rather than miles. If you don't mind spending double on the cost of oil go with synthetic 10W-30 or even 5W-30 (synthetics don't loose viscosity much as they wear out).

Classic Miata and obviously driven little and well kept, great find!

This message has been edited. Last edited by: ridewv,

I ran your car through the Mobil 1 oil finder, and it suggested 10W30 and Euro 5W30 FS. I think the Euro 5W30 is a great year round choice.

The use of 5w30 synthetic has obviously not harmed the car. You can keep using it with confidence.

quote:

In the pic, it seems the factory engineers were well aware of 5W30 ...

I'm pretty sure 5W30 existed for a couple of decades prior to the 1990s.

I would use synthetic 10W-30, change every 3k miles and not worry about it. This guy has a lot of info on oil.

Motor Oil Geek

I changed oil every 7500 miles. I think 3000 miles with synthetic is overkill but its your money. Like I said 250,000 miles with a steady diet of red line shifts and it still had good compression.