CentralCoaster

From my '85 GM manual:

What's the deal with that? It seems ludicrous that they'd be so insistent against it. FWIW, I've been using it for 8 years on all my cars.

bogus

modern engine tolerances are so tight, that it's possible that 10-40 might not make it....

I ran into some kid last year at the Sebring 12 Hours with a brandy-new Evo VIII. Bad-assed car. It was NEW - no miles yet. He could not wait to change the oil from the factory 0-30 fill to 20-50. I told him that the internals are so tight that 20-50 more then likely would cause oil starvation.

I am repeating that here... I suspect that if there had been an engine failure, and you were using 10-40, GM would void the warrantee.

93 ragtop

Some of the new fords and hondas require 5-20 oil. And Mobil's replacement syn. for the hondas is 0-20 which is supposed to be approved in the hondas.
IMHO where the damage would or could occur is during cold start up. If the oil is thin, it will get to the bearings quicker then a thick oil.

mikenfl

There are several points to consider here. SAE 10w-30 and 10w-40 are both supposed to flow the same when cold (at start-up). On the way to operating temp, the 10w-40 doesn't thin out quite as much. I know, I have a firm grasp of the obvious. Could this difference cause a problem in a tight engine? Possibly yes. However, we're not dealing with new cars here. Some of us may have less than 50,000 miles racked up, but many have alot more showing on the odometer of our beloved C4s. Our clearances probably aren't quite the same as they were when they rolled out into the sunlight at Bowling Green. Lastly, one of the most important reasons GM specified 10-30 over 10-40 was, and still is, fuel mileage.The small difference it makes may seem insignificant, but in the quest for better corporate numbers, it matters. Of course, this is all just IMHO and may really be

rws.1

It could also be that in '84 when the manual was printed that 10W40's were not that common and the few that were available did not stay in grade as well as 10W30's that were available.
When any oil is subjected to sheer forces in an engine it sheer thins , when it starts getting laden with combustion by products it is subject to oxidative thickening. Keeping the oil within its designed operating viscosities is the job of detergent and viscosity index improver packages.

My recollection is that in the early days of 10W40 motor oils that the VII packages were not robust enough to keep them in grade and the 10W40's quickly sheered down to 10W20's ....BUT that recollection may be faulty and or if accurate may not fit the time period.


The engine is not now as tight as it was then and motor oil technology has moved on. The 10W40 may not be the optimum oil for fuel economy or for engine longevity, but if faced with dirty oil or no oil, I wouldn't hesitiate to put it in.

The question is why do it? If 10W30 is recommended , is readiliy available in a number of different formulae ( new car, high mileage etc)
in conventional, re-refined, and synthetic AND is cheaper ... why use 10W40.... ?

rick lambert

in fact, I would think on a car with 50k miles on it could use any engine oil with a 10W40 viscosity since the clearances have undoubtebly increased.Years ago we always went to higher viscosity ratings when the engine was getting a little noisy and oil pressure was low. Checked my 87 service manual and I can't find anything relating to a specific viscosity-think for kicks I'll look at the owners book that came with the car.

On the other side-you still have the sticker that says "unleaded only".

kwik_ta

Bogus is right as usual... I bought my 94 Cougar from my dad (who bought the car new) some idiot told him that after the car passes 125k it is best to use a 20-50 racing oil because it coats the parts better... so he did. I advised against it. Later he noticed a considerable drop in oil pressure and various problems arising with the car so he decided to sell it. I bought it, ran some Lucus and Castrol 10-30 in it and it is pulling the same pressure now as when it was new.

CentralCoaster

I should have posted more info, but the 'too thick' answer doesn't sound right.

I think RWS is on the money.

The other oil the book specifies are 5w30,15w40, 20w20, and 30, depending on temperature range.

So there's a 40 weight, which only matter in hot temperatures. But I noticed the largest octane split on here is 25w, whereas 10w40 is more than that. A whopping 5 weight, but oils have probably improved since then anyways. I guess what it shows me is 10w40 isn't the best choice, I think I'll switch to 10w30.

As far as modern engine tolerances, Bogus, this is a Gen-I small block with forged pistons. Don't forged pistons = worse tolerances? Obviously manufacturing tolerances can still change with the same part#, i dunno. It'd be nice to find those tolerances somewhere to see when in general, they improved.

Thanks everyone.

CFI-EFI

I checked my Service manual after reading this and find no such warning. In fact there is a chart on the oil to use based on the expected temperatures. 10W-40 is included as recommended along with 10W-30 from 0*F to over 100*F. They even list 20W-50 for 20*F and hotter. This is especially interesting because my manual is for only one year earlier than CentralCoaster's. I've been using 10W-40 for a couple of years to help slow the hemorrhaging past the rings and valve guides of my high mileage, highly abused engine. Now that I've checked the manual, I may go thicker.

RACE ON!!!

TIMSPEED

Funny thing...
I was looking back through the reciepts, where the PO (Original Owner) had all the oil changes done @ GM, and EACH TIME they put in 20-50!! I was like

bogus

I agree with the contention that mileage would have a bearing here.... but the piston fit isn't the real concern, it's ring tightness.

However, if oils had an instabilty problem, then that would also add to the equation.

Whatever the real cause, I suspect it's been hit here rather thoroughly.

nel

Back in the '70s and '80s, auto manufacturers ran into some real problems with 10W40. In order to get that viscosity spread, oil companies had to use a large amount of viscosity index improvers (basically a type of polymer that changes in shape with temperature). These VII's would shear during use and end up creating sludge.

Nowadays, non-synthetic 10W40's are much better constructed. However, they still shear but it's not as bad. If you need a 40wt during the summer, I'd recommend using something in a 15W40 or a synthetic 10W40 or 5W40.

ClarenceT

The SBC is a dinosaur, has nothing to do with 'modern engines'

My dad runs straight 40 weight in his vette. It's garaged, and we're in Southern Cali, where the lowest temptreture at night in the dead of winter is 54. His is a '71 that he bought new, now with 98k miles on it. It still runs damn perfect on the same motor (seals had to be redone, I think he did the timing gears+chain while he was there, but that's about it)

I honestly doubt anything bad will happen if you run 10-40, even if you're in alaska

_twisted_

It's all about the polymers used to get that viscosity, If you pull a V/C off an early 80s or prior SBC you'll find sludge and all kinds of gack, clogged/restricted oil passages. In a motor that has seen 10/30 or 5/30 it's scary how clean a 100k motor can be.
The switch from 10/40 will be like putting your baby on a low cholesterol diet.

Good example (wish I had pics) me and a coworker bought trucks identical except for color within a month or 2, he was a hammer head and had to use 10/40 cuz Daddy told him too, I used 10/30. Neither one went more than 4k between changes, one night into work he lost oil pressure at lets say 90-95k, turns out his screen was all sludged up and not letting oil flow freely. I sold mine at 148k and never had to drop the pan and clean the screen. Oh I forgot to tell ya, the hammer head also only ever ran 5qts instead of 6, until I told him and then he verified through the dealer ( in defense of hammerhead, he wasn't the only Ford owner I know to run the 5 qts) .
So you think it was a combination of always being a qt low and the weight or just the weight?

bogus

It's original design is a dinosaur... but the modern construction is very modern indeed. The metalurgy and precision fitment of components has improved exponentially from 1955 to 1975 to 1985... and on to 1995. I would love to see the specs for tolerances in a 1955 265ci SBC and the ame specs for a 1995 LT1. I bet there is a TON of differences.

Corvette Kid

[QUOTE=rws.1]It could also be that in '84 when the manual was printed that 10W40's were not that common and the few that were available did not stay in grade as well as 10W30's that were available.
When any oil is subjected to sheer forces in an engine it sheer thins , when it starts getting laden with combustion by products it is subject to oxidative thickening. Keeping the oil within its designed operating viscosities is the job of detergent and viscosity index improver packages.

QUOTE]
And viscosity, from all I've read has NO effect on film strength. I'm using 15-50 M1 in warmer weather BUT my clearances are looser (by design) than a stock LT1 because I built it for hard running. Smokey Yunich always said that assuming a properly designed and functioning oiling system, that as a rule of thumb, 10 psi of oil pressure per 1,000 RPM was all that was needed.

Corvette Kid

I should add, for a stock engine or even a modified one with stock bearing clearances, I too would stick with 5-30 or 10-30.

tonymax2

That's true!

Hotred94

My mechanic agrees with Nel's viewpoint--too many additives in it to extend the range; the hydrocarbon chain is too big to get down into tight spaces. He always recommends 10-30 or synthetic.

CentralCoaster

Duh! I knew that. :o