Oil too THIN for warm up?

[n8wvi]

Ok, now that I've got your attention, let me clarify, I said warm up, not start up. If start up is the 3 seconds after you hit the key, and warm up is from the end of start up and continues for 10 minutes 'til the oil comes up to temp. Here's why I'm asking. My MIL's Miata calls for 20w-40. I've been running 15w-40 for the last 2 years, but I'm considering going to 5w-40 (Valvoline PBE), then I wondered why Mazda ever recommended a 20w-40 anyway. Is it possible that during the WARM UP phase, they want something thicker in there (yes, I know a cold 5w is much thicker than a hot 20w, but a cold 20w is even thicker, and that's what they've spec'd).

Any guesses on the reasons for the crazy spec?

Dave

 

[Curious Kid]

FWIW - my Grand Mother's '77 chevette with the little 1.6 four could take 20w-50 during the summer months as listed in the manual. If anything, I thing it has to do with making sure the oil retains some protective viscosity over the OCI. Back in the late seventies I bet the concern was shear thinning, as even some mention concern with today's oils even. Small displacement, high sustained loads all in a compact design...ouch!

What year is the Miata/publish date of the manual?

Honestly, I don't see any other concerns especially if oil pressure looks good. The recommendations in the manual concerning oil viscosities are often a good start as many mention around here, and will change with driving conditions (ambient temp, engine loading, etc.).

 

[BlazerLT]

In michigan, I would use a quality 5w40.

It is the second number you worry about.

The first is for the cold temperature pumpability and the lower the better.

 

[steveh]

Is the car a 1990? I think the specs were changed for 1991.

 

[n8wvi]

Yes, it's a '90.

BlazerLT, I know the 'second' number is the only one that matters when the oil is warmed up, but why would Mazda specifically recommend a 20w for the first number???

Dave

 

[Gary Allan]

Maybe because 10w-40's were so lame at the time and didn't remain 40 weights very long

..and that the use of synthetics, in engines that didn't come from the factory with synthetic oil, is never spec'd as a "must use" oil ...hence the most stable 40 weight PCMO available was spec'd. (although the only time I've seen 20w-40 is in a fleet type oil

)

 

[TooSlick]

For the same reason why Mercury Marine makes an SAE 25w-40 for their marine inboard engines....

This is an "old school" recommendation and they expected the conventional multigrades of that time period to shear down in viscosity....

I'd run 0w-30 or dare I say it 0w-40 up in the Great White North....

TS

ps: Good for Mom I say! - just watch for the blue hair and the cruising of Caddy dealerships when she gets older...