Operating viscosity vs. starting viscosity - ??

[addyguy]

Okay, I'm not sure if I'm going to get this 'right', so bear with me....

Owner/service manuals for my car (2002 Cavalier) say, for oil viscosity, 5w30 preferred for all temps, 10w30 acceptable for 0F and up. If it is an 'emergency' and neither of these grades is availiable, they alow the use of straight 30-weight for temps above 40F. All these oils have in common is that they are 30-weights at operating temperature.

If you 'had' to use straight 30 near 40F, it would be pretty thick, and you might get a bit more start-up wear. I'm suprised that in 'emergency oil situations', GM doesn't recommend using an oil like 10W-40, so you get SOME start-up protection at borderline temps. But GM absolutely forbids the use of anything outside of a 30-weight.

To me, this means that if 'push comes to shove', GM is saying that the 'operating' temperature of the oil is more important than possible start-up wear cause by using a straight 30-weight. I know they clearly state that 30 weight is only to be used above 40F, but I'd think they'd err on the side of caution, and recommend an oil that has more 'start-up' protection, ie, a 10W oil, than stick to wanting the fuel economy/operating gains given by a straight 30-weight. If a person got stuck in a 'cold situation', they'd be better off with a 10W-40, and be able to start their car, even if they DO get slightly lower mileage...

Am I totally off base here? Anyone else agree?

 

[FrankN4]

I know it has to do with their CAFE regulations, but, I don't think they would/could recommend a grade of oil heavier than 30 under any circumstances. Also, if they said you "could" use a 40 grade oil they would open the door to your using a 40 grade all the time and they would still be required to honor warranty claims.

I have a 1999 Cavalier I bought for my wife in Decembe 1998. We drove it home from the dealer and changed the factory oil to Mobil 1 15W-50 while it was still kgood and hot. It now has 206,600 miles and has had nothing but Mobil 1 15W-50. It doesn't use a drop, doesn't leak or seep, and gets 28 mpg around home and 32 mpg on a trip.

 

[Steve S]

Where you will get into cold start wear is when the oil takes a long time to get to the pump when the wrong viscosity oil is used for the ambient temps for example a straight 30 in 20*f think of a real thick milkshake sucked up through a straw.10w-40 oil has a potential of sticking the piston rings with deposits .Steady state marine ,natural gas ,diesel engines mfgs recommend a straight visc. oil for that reason. A person would be better off sticking to recommend oil viscosities.10w-40 most likly doesn't have any advantage over a 10w30 due to the higher viscosity improver levels which may be the reason that the 5w-20 oils are working as good as the 5/10w30 oils in many situations.

 

[Zaedock]

I would tend to agree about 10W40 vs SAE30. I will say however, that the Delo SAE30 I have used in my Jeeps flows pretty well down into the low 30's. The Delo pour point (-24*F) is on par with most 10W40's I've seen. I've done some research on the Delo SAE30 because I have so much of it (remember that $.99/gal sale back in fall '05?).

If I was ever in a situation as described though, I would use 10W40 over typical SAE30.

 

[XS650]

A comment for those considering using a straight 30 weight oil. Not all 30s are as good as the Delo at low temps. Be sure you know what low temperature performance is for the 30 weight you use if your weather gets below about 35F.

Some straight 30 weights would qualify as 20W-30, 15w30 or even 10w30 if they were tested for low temperature properties. Others wouldn't qualify for any cold test W rating.

 

[tom slick]

In the 90's GM was adamant against using 10w-40 in anything because of the sticking ring issue Steve S mentioned. That could be why they still don't recommend anything over a SAE 30.

I think the emergency recommendation is comical. if it really an "emergency" would you really care what you were putting in?

 

[addyguy]

XS650 - I agree entirely, that a lot of 'modern' straight 30-weights are actually closer to a 15w30; and actually do have good enough cold proerties that they could be used much, much below the rec. threshold.

Steve (and Tom) - Yes, ring sticking was an issue with 1970's-era 10W-40's. But we all know on here that that is not an issue anymore, as the quality of 10W-40's has improved dramatically. GM engineers should know this, and know that today's 10W-40's are not a peroblem in this one specific area.

Tom - this is just a theoretical question I am putting out. Of course I am aware that if it was a real emergency, what oil is actually in the crankcase of a car is of almost no importance....