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Oil viscosity...does it matter?

I have always changed my own oil. However, last year I bought a 1990 Miata (80,000 miles) and I do not know how anyone gets to that oil filter. So I have been taking it to the dealer for oil changes. The owner’s manual recommends 10w30 in colder months and 10w40 for warmer months. When I requested 10w40, the dealer said that they only carry regular 5w30 and 5w20 synthetic blend in bulk. I would have to pay $10 extra for the old 10w40. My question…will the more modern (thinner) oil work well in a 1990 Miata, or should I stick with the owner’s manual recommendation?

5-20 works fine in the winter and 10-30 in the summer. When it REALLY gets hot, I like the 15-40 fleet oil which is rated for both gas AND diesel engines. This grade of oil also answers the cam and tappet galling problem found in some of the new low additive, converter safe, EPA approved products now on the market.

NOBODY takes there car to a dealer for an oil change…If THEY can change the filter, so can you…

Stick with 10W40 or even a little bit thicker as the engine ages. Ask the oil change place if they’ll do the labor at a discount if you bring your preferred oil. Some might, some might not.


An oil filter is typically good for 10,000 miles or more. Have your shop hange it in fall when you switch to 10W-30 but keep the same oil filter in spring when you change the oil yourself to 10W-40.

I had a 1990 Miata and as I recall I used a strap type tool to remove the oil filter. It was not much of a problem and I did it often.

It was a great car.

Always change oil filter at every oil change. Over time, the filter get saturated with dirty oil & won’t filter any more. If you don’t change it, your new oil will just go in & out the filter w/out any filtering. Oil filters are cheap, around $3.00 to $5.00 for a regular one. You decide if you want to save $3.00 to $5.00 or $200.00 or more in repair bills!

You use a strap wrench It looks like a stick with a cloth strap on one end.  You can find them at any auto parts store.  I might add that if you have long skinny fingers like mine it makes that job a lot easier.  My Miata was the tightest fit on a oil filter I have had. 

As for the oil, you should be able to buy the oil you want, and provide it to the dealer.  BTW why the heck are you going to a dealer?     Dealers are no better (or worse) than independent mechanics for almost anything you might need done on your car.  They will almost always charge more per hour and often more for parts and supplies.  They also tend to look at repairs a little different than the independent. 

A dealer may well recommend work that strictly may not be needed, but could be connected to the problem or maybe replace a part when a little repair would fix it ALMOST as good a new.  

There is no need to bring your car to the dealer for any service other than service that is going to be paid for by a recall or original warrantee.  During the warranty period be sure to document all maintenance work.

I suggest that most people would be better off finding a good independent (Not working for a chain) mechanic.

Greg, as a dealer technician,(not Mazda) I am surprised at your dealers attitude on lubricants. The oil recommended in your owners manual is the oil you should use. There may be a chart for oils to use at different ambient temperatures in your manual. Thinner oils are only required if the ambient air temp is below about -20 degrees (f). Heavier oil is not required until about 110 degrees (f). I would also urge you to continue to change your own oil. You will save yourself a bunch of money in the long run.
And be sure to change your filter at every oil change. You wouldn’t get out the shower and put on dirty socks, I hope.

Where’s the filter on the Miata? I’ve done oil & filter changes on lots of vehicles and have never been unable to “get at it”. Many times I have had a stuk filter (whomever put it on really tightened it too much . . . or it was on too long . . . but that’s not your issue . . . you say you simply can’t get to it? On the oil . . . use the weight recommended in the owners manual . . . when you get high mileage try a little thicker. Rocketman

I’m not sure why you can’t get 10W40. I buy it all the time. What I buy is not part-synthetic; is that your concern? If it is, it shouldn’t be, because on a 1990 car, depending on the car, it’s possible you should not run a synthetic. I’d consult the manual on that one.

Just glancing through Castrol’s website, you can get pretty much any oil you’d want from them in a 10W40. I believe your garage owner is just being a little lazy. 10W40 is considered a little heavy for most newer cars, and he probably thinks it’s not worth stocking oil that will go into a dwindling number of cars that are over 10 years old, which is a shame because I think that Miata is built to last.

I find 10W40 Castrol GTX at the ultra-exotic Kragen about 1.4 miles from my house. My 1996 uses the same oils as yours: 10W40 in the summer, 10W30 in the winter.

I think 10w-40 oils have changed over the years. But for a long time that weight oil was NOT recommended my several manufacturers. 10w-30 or 10w-40 both are 10w oils with additives that increase viscosity as the temps raise. To create 10w-40 it takes more additives then it does to create 10w-30. The additives are NOT good lubricants. For years the recommended range was no more then 20. Any more then that and there were just too many additives, thus decreasing it’s effectiveness. Now with that said…I think there have been some major improvements in oils and I heard that the problems they had with 10w-40 no longer exist. I really don’t know. I’ve been using 10w-30 for years and several hundred thousand miles.