Do Not Use Sae 10w-40 Oil
 

DO NOT USE SAE 10W-40 OIL That is what the 1985 Corvette Users Manual says in BIG BOLD LETTERS.

5W-30, 10W-30, 15W-40, 20W-20, and 30 are all OK, but not 10W-40. Why not? I've been using Valvoline 10W-40 for 30+ years and this is the first time I've seen a warning like this. :eek:

:lurk:

Perhaps...
 

It seems like I read here somewhere that it had something to due with vicosity breakdown(back in 85). 10w40 as i recall, was not very stable back in 85. As they have inproved the formulation over the years, stability has increased.

I was surprised to read the same thing. :smash:

10/40 is all I use in my 89 (Dino oil), an same grade Mobil 1 in the Z?!

Covers too wide a viscosity range. Jack of all trades master of none. Notice that all the rest have a smaller range.

Google is your friend. I'm sure this is in every oil FAQ out there.

You musta missed this thread, similarly titled:

"Do not use Sae 10w40 oil"

http://forums.corvetteforum.com/show...m_id=48&arch=1

If it was the logic of the point difference (or spread) than 0-20 on a motor tight by design would not be good either. Additives to support weights is a possibility also, but for the most part I believe it is about clearances, therfor the nessecity to get the oil to lubticate everything. New production (last 15 years on so) tolerances are different than motors that were built in the 60-70's.

Odd. I did a search before I posted. I must have done something wrong because this didn't come up.

First, I can't find such a statement in my Service Manual, GM ST-364-85. I recall reading somewhere about " molecular strings" in oil. Larger the spread,10-40, longer the strings and more subject to break down.
I will not argue with the experts but 10-40 hasn't let me down for more than 40 years so I'll stick with it. I am building a 383 to go in my 85. It will live on 5-30 Mobile 1.

Bottom line guys! Both this and the old thread dances around the issue, sometimes touching it but never quite right on.

All 10w40 oil was recalled about the time of the 85 cars were introduced due to failure in engines. It had been out for a while and there was tons of it on the shelves of every kind of store from the 7/11's, to the grocery store, to the big auto parts stores across America. Auto manufacturers were well aware that the recall would fall on deff ears at most places except for perhaps the big auto parts stores and gas stations, and they didn't want to run the chance that some of the stragler packages of 10w40 might be aquired and put in their cars. Especially since the Oil companies were trying there best to keep the recall quiet, if for no other reason, to avoid everyone in the U.S. who has an engine burn up, blame them and sue.

The cause of the failure was the newly developed pollimer package which had been developed to produce that 30 point spread in viscosity.
The new formulation had flaws and broke down under certain situations and lost it's lubrisity, and allowing metal to metal contact.

I can only assume that these problems have been long solved are the oil company's would not have put it back on the market and invited further suites.

However, to this day I will not use a 10W40 oil in my car.

5W-30 tells you the oil's thickness, or viscosity. A thin oil has a lower number and flows more easily, while thick oils have a higher number and are more resistant to flow. Today’s High performance engines have such tight tolerances that you need slightly less viscous oil in order to properly lubricate all of the critical parts.

Feel free to use what ever oil you want, but don’t be surprised when your engine shows signs of premature wear.

As far as using a product for 40 years 40 years ago people thought smoking cigarettes was good for you times change.

Ok everyone. Thanks for responding. I think next time I change oil I will go with the Mobile I 10W-30. I'm already using their filters.

After my engine had over 100K miles, it didn't bother me to put 10-40 in only because the tolerances had increased due to wear. I still use it in some of my cars.

Art

This is what I understand as well.
I've read that more than 20point spread on a multi-viscosity oil means the oil is not as "stable", or something along those lines.
This was the current thinking as of ten years ago ....it may all be changed by now. :crazy:

Larry
code5coupe

I run 0-40W Mobil1 in my 89 coupe that has a 383 under the hood :cheers:

don't know the ''theory'', but...

long long ago, bought a new 77 caprice...first oil change put in 10-40...car used a quart of oil every 1k for months...went back to dealer and was told not to use 10-40...changed to 10-30 and oil consumption ceased completely