[MSCW list] Oil Viscosity - NA Miata

[Timothy Metzinger]

I run M1 5W-30 year round.  And you should check the pressure with a mechanical gauge - after being at operating temp, it should be 28-43 PSI at 1000 RPM (that's NOT at idle), and should be 43-57 PSI at 3000 rpm.
My factory gauge reads low, but my aftermarket gauge will show good numbers with the 5W30.  If you want to come by some time and borrow a mechanical pressure gauge, you can.

My 91 Factory Shop Manual specifies 30 weight (page D-3).  Since it's the same engine with the possible exception of the crank nose, I'm sure it's fine.


-----Original Message-----
From: MSCW [mailto:mscw-bounces at lists.mscw1.com] On Behalf Of Jack McCombs via MSCW
Sent: Monday, June 26, 2017 11:31 AM
To: mscw at lists.mscw1.com
Cc: Jack McCombs <jmccombs3 at aol.com>
Subject: Re: [MSCW list] Oil Viscosity - NA Miata

<<<<<  www.MSCW.com  Mazda Sportscar Club of Washington (DC), Inc.  >>>>>
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I took our '90 Miata in for an oil and filter change this morning.  The shop (Hogan's Goodyear, Fair City Mall on the east side of Fairfax) had nothing heavier than 10w30 and 5w30 oil.  The manager insisted that the heavier oils specified in the manual were not right for the car. (!)  I checked my owner's manual beforehand, because this shop had installed oil that was of too low a viscosity in the past.  With the lighter weight oil, I found my oil pressure gauge reading down around 10 psi at idle when the engine is at normal operating temperatures on a hot day.  Yet they insisted that they always used the proper oil as specified by the manufacturer.  Bull.  I canceled the appointment and scheduled with another shop (Main Street Shell, Fairfax) that said they had higher weight oils.

My owners manual specifies straight 30, 10w40, 20w40, 10w50, or 20w50 for ambient temps around 90 deg. F., with 10w30 being OK only up to 90 deg. and straight  40 above 90 deg.  I'm going with 20w40, since this second shop says that they stock it, although straight 30 would also be OK.  10w50, 20w50, and straight 40 are probably a bit extreme, viscosity-wise, but what are others using at this time of year?  The car is high mileage (355,000), and tolerances have loosened up a bit over the years, as it goes through on the order of three quarts or so every 5,000 miles.  Hey, it's still cheaper than buying another engine.

--Jack McCombs & KANSEI ('90 Miata)--
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