Selecting engine oil based on temperature

[mdwilson]

In the past, I frequently saw API temperature/viscosity charts in auto repair books. These charts showed which engine oil to select based on the range of temperatures where you were doing the driving.

Back when I used to see these charts, 10W40 oil was very popular. Many of the charts contained a warning such as "Note that 10W40 is not recommended for sustained high speed driving above the indicated limit!" It was not recommended for use in warmer climates, and a high heavier oil was a better choice (15W40, 20W50.) Driving down I-95 to Miami in the summer with 10W40 was a bad idea because it was not heavy enough for the temperatures.

Nowadays, this chart is nowhere to be found. All the newer vehicles seem to come with a recommendation for 5W30 or 10W30 oil. I don't recall the chart exactly, but I know 5W30 used to be recommended for nearly Arctic conditions! Think North Dakota in winter. Now it is a blanket recommendation for cars, even if you are driving during summer.

What is going on here? What changed over the past 20 years or so regarding oil, or engines, such that a lubricant that was once considered completely inadequate for hot weather usage is now the recommended norm?

 

[dgallup]

Fuel economy requirements happened.

 

[mdwilson]

So, if you disregard the manufacturer's concerns on meeting fuel economy standards -

I should still be using the heavier oils at higher temperatures for the best mechanical protection?

 

[ivymike]

and using synthetic ones too... but I'm not sure you'll see an important benefit in terms of engine life.

Piston rings tend to have much better face coatings than they used to, so they'll last a good long time with the lower viscosity oils, and maybe even a bit longer with thicker ones. Piston skirts will wear less, but weren't going to be the limiting factor.

Valves don't get terribly much oil to begin with, and using a higher viscosity will likely reduce the amount they get.

Hydraulic lash adjusters (if so equipped) may run a bit on the tight side if you use a higher-viscosity oil, which could lead to problems on a cold day.

bearings will wear less with the higher viscosity oil, but probably weren't going to be the limiting factor in the first place.

Your cams probably won't be noticeably different in the areas that wear, because the oil film is pretty darn thin there anyway (and the additives make a bigger difference).

Your head gasket isn't going to care, and your head itself won't live any longer... the accessories won't be helped... and your car will probably be crushed in a wreck before the engine is used up anyway.

 

[evelrod]

I agree pretty much with your assessment, Issac. However, there is one bit that need a little clarification...

"bearings will wear less with the higher viscosity oil, but probably weren't going to be the limiting factor in the first place."

With the newest engines bearing clearances set closer than in the past (due in part to NVH requirements according to Greg) and using a "thicker" lube could result in MORE wear, not LESS. Perhaps even total failure? In some of Ford's literature there was an admonishment about using higher viscosity engine oil in place of the 5W-20 synthetic (for our Lincoln). I have no direct comparisons but I would like to know for sure. The Lincoln has lived through our 110+f summers now for nine years and 169,000 miles and is still "as new" by all considerations....Yeah, I know I too cheap to buy a new one....Next year, maybe!

Rod

 

[mdwilson]

OK, another way to approach this - what the heck is all that higher viscosity oil being used for?

Plenty of 10W40, 15W40, 20W50 for sale...are there any cars that aren't calling for 5W30 or 10W30? Or are people just ignoring that and using heavier oils like always used to be the case?

 

[evelrod]

Well, I'll stick with the 5-20 from Ford for mom's car but we use Redline 50 in the Mini and Redline 40 in the Lotus race cars.
Castrol 20w-50 in the Cummins/Dodge (200+k) so I can account for some of it. Also use 15W-50 Mobil 1 in the fully inserted 1930 Model A. 10W-30 in the 454 motor home. 20-50 in everything else as I built in ~.002" bearing clearance in the engines.

I suspect your second sentence is the correct reason. At least that is what my dad would have done. Of course he thought the moon landings were filmed in Hollywood...

Rod

 

[Tmoose]

There was for a time a decade or 2 ago that GM specifically advised NOT to use multiweight conventional oils with a range greater than 20 to avoid problems with some kind of deposits. 10W-30 was OK, 10W -40 was not.

Years before that the long chain molecules responsible for the high temp viscosity rating in multiwights reportedly got mulched in use, and over time the oil viscosity became lower, more like the base oil. That was reportedly why manufcturers like Porsche and Harley refused to jump on the multiweight wagon for years.

I expect that neither of those are much of a problem today

 

[rmw]

I thought IvyMike was going to say 'besides your car will be crushed for the recycler before the engine is used up anyway.'

rmw

 

[Tmoose]

Or, the engine will be murdered by lethal injection in the next cash-for-clunkers program when all the cars getting less than 65 mpg are sent to the showers.

 

[drwebb]

Passenger car engine oil performance specifications have gone through 4 (soon to be 5) category iterations in the past 20 years. An API SG category 5W-30 is probably not suitable for modern equipment calling for an SM category 5W-30- or maybe even a 5W-20.

FE benefits and the fact that modern equipment doesn't normally 'need' a thicker oil are the main reasons for the OEM recommendation side of the shift. There is probably also some commercial influence as well. 1999 North American refinery capacity to produce heavier oils suitable for heavier engine oil grades (Group I) represented 51% of capacity. By 2009 that had dropped to 30%, while the more economical and higher quality Group II oils grew from ca. 28% to 49%. This has made some of the ingredients for heavier grades more costly, so oil marketers prefer to market lighter grades as well.

Certainly there have been a lot of changes to PCMO specifications and recommendations in this time, and conscientious car owners may have concerns. Consider that recommended drain intervals have gone up as well- reflected in the field by average US oil changes which decreased every year from 3.4 in '03 to 2.8 in '08. But with all these changes, is there any indication of a degradation of either average engine service life or failure frequency?

 

[ivymike]

none that I'm aware of, but the assessment period is not yet adequate to judge the results.

I'm a bit surprised that a car "designed" for 5W-20 would react so poorly to a 10W-40 (as in Rod's comment above). Would that be attributed to inadequate lubricant supply (mass flow)?

 

[evelrod]

I doubt a 10-40 would do it, Isaac. Perhaps they were thinking along the lines of a 20W-50. Personally, I have never had any problems along these lines, but, I don't live where it snows.
Yeah, I can see the potential for problems with cold engine/thick oil in combination with idiot driver.

Rod

 

[TheBlacksmith]

Whilst I don't have the experience of some of the posters, I live in coastal Virginia where it goes from near zero to over 100 and have driven from upstate NY (below zero) to FL. 1998 Saturn 5W-30; running great at 132,000 when the youngest totaled it, 1999 F-150 5W-30, still going strong at 163,000, 2008 Miata 5W-20, running great at 25,000 when the ex took it.