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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.
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.
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.
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. .
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. Todayís 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 donít 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.
Location: SCMR Rat Pack'r Charter Member..Great Bend KS
Originally Posted by Vis Croceus
Covers too wide a viscosity range.
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.
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