oil viscosity for 67 300 hp in South

[William F.]

Duke et. al.
I have access to Rotella -T in 30, 10w30 and 15w40 . Summer highs get into the 90's: winter time we do have occasional freezes and highs in 40-50's range. If I'm willing to change oil in fall and spring, should I run 30 in summer and multi vis in winter or multi vis(which one?) year round?

[Duke W.]

Re: oil viscosity for 67 300 hp in South

As is said in the Summer 2008 Corvette Restorer engine oil article, 15W-40 is suitable for cold starts down to about 10-15F.

A high temperature viscosity rating of 30 or greater is suitable for summer ambient temperatures equal to the highest you will find on planet Earth.

So unless you do cold starts consistently below 10-15F, 15W-40 is suitable for year round use.

For those of you who store your cars in subzero winter temperatures, viscosity is of no consequence during storage. The low end viscosity is chosen based on the lowest anticipated cold start temperature, not storage temperature. If you never cold start below about 40F a straight SAE 30 would be okay.

Duke

[William F.]

Re: oil viscosity for 67 300 hp in South

Thanks, Duke
Bill F.

[William F.]

Re: oil viscosity for 67 300 hp in South

Duke,
One more oil question: Since I can get either 10w30 or 15-40 Rotella T, which would be better, rembering I'm in a hot climate. Original owner's manual specified 10w30 for a multi vis but does it thin out too much? On other hand, is 15-w40 too thick to get into small tolerance areas?
Thanks,
Bill

[Duke W.]

Re: oil viscosity for 67 300 hp in South

I use and recommend 15W-40 C-category year round for vintage (and "older cars" with sliding surface valve trains) cars that are not cold started below 10-15F.

That's what I use in all my cars, '63 to '91. Where I live a mile up from the beach in Southern California the temperature rarely gets below 40F.

Prior to the reduction in P that began with SL I used 20W-50 S-category oils that were dual rated. They carried the then current C-category as a secondary rating and had about .012% P.

Since 15W-40 and 20W-50 were not commercially available in the C2 era they are not listed in the owner's manuals, but 20W-50 is listed in seventies GM owner's manual as suitable for 20F and greater; 10W-30 is listed for 0F and up, so i split the difference for 15W-40.

Also, the seventies GM owners manuals say that straight SAE 30 is suitable for temperatures of 40F or greater.

You could use any of the above in your climate, but I would choose 15W-40.

Duke

[William F.]

Re: oil viscosity for 67 300 hp in South

Duke Williams,

Hate to bug you again about oil viscosity for my 67 300 hp, but-I know 10w40 and15-40 was recommended for later engines but 67 owner's manual recommend nothing greater than 30 or 10w30 and then only for
'high speed driving or temps 90 or higher".
Would 15w40 possibly be too thick to lubricate the close tolerance areas or is this not a problem with an engine with some miles on it in ahot climate?

Thanks,
Bill Ford

[Duke W.]

Re: oil viscosity for 67 300 hp in South

No, the 40 high temperature viscosity rating will not cause any lubrication problems. As I said, I used 20W-50 in all my cars from the seventies to the late nineties in my mild California climate.

Duke

[Michael H.]

Re: oil viscosity for 67 300 hp in South

Duke Williams,

Hate to bug you again about oil viscosity for my 67 300 hp, but-I know 10w40 and15-40 was recommended for later engines but 67 owner's manual recommend nothing greater than 30 or 10w30 and then only for
'high speed driving or temps 90 or higher".
Would 15w40 possibly be too thick to lubricate the close tolerance areas or is this not a problem with an engine with some miles on it in ahot climate?

Thanks,
Bill Ford

I'd go with the 10W30, just like the manual suggests. I tried 15W40 in my 66 and the oil pressure was too high.

Thicker ain't necessarily better when it comes to oil. I think the guys in GM engineering knew what they were doing when they made this recommendation.

[Stuart F.]

Re: oil viscosity for 67 300 hp in South

Michael;

But that recommendation was made long ago. Things have changed quite a bit since. I agree the pressure is higher, but I'd rather see that then it fluttering at 15# idling in summer traffic.

Are you just trying to pull Duke's chain on this? Michael; it's a nice day - not too hot. Go out and work on one of your cars. Change the oil or something.

Stu Fox

[Michael H.]

Re: oil viscosity for 67 300 hp in South

Michael;

Are you just trying to pull Duke's chain on this?

Nope. It has nothing to do with Duke.

With 15W40 Rotella oil in the 66, the oil pump GROANS when started cold and the oil pressure is around 70 PSI at idle. At 1500 RPM, the ga is pegged!
It stays that way for at least the first 10 minutes of operation.
When at operating temp, the idle pressure is around 35 and it goes to around 80 at 2500 RPM.

In all the 25 years that I've owned the car and used conventional 10W30 oil, the cold start idle pressure was around 45 and hot idle pressure around 20.
Hot high RPM pressures were reduced also but I don't remember the exact numbers.

If people want that kind of oil pressure, that's ok with me. I don't though. There's absolutely no need for it.

I rebuilt that 425 HP in about 1983, driven it all around this country, and it hasn't been apart since. I must be doing something right.

I can give you a lot of technical reasons why oil viscosity is critical but most won't understand it. It has to do with it's "sheer" properties.

If thicker is better, shouldn't we just use 90W gear lube?

New cars survive very well with about half of the oil pressure that cars of the 50's-70's had.

[Duke W.]

Re: oil viscosity for 67 300 hp in South

Some of you need to go back and look at the Web references in my oil article from the Summer 2008 Corvette Restorer, specifically the engine and gear oil viscosity range comparison chart at 100C:

http://lubricants.s5.com/viscosity_c.htm

1. The total range of SAE 30 through 50 absolute viscosities is only about 2:1

2. Engine oils use SAE viscosity grades from 10-60, and gear oils use 70-250 in order to distinguish them from each other. However, an SAE 75W gear oil has about the same absolute viscosity as the SAE 10W-20 engine oil range, and a SAE 90 gear oil overlaps the SAE 40 through 60 engine oil ranges.

So a statement like "why not use SAE 90..." indicates an fundamental lack of understanding of the SAE viscosity grades used for engine and gear oils, and a fundamental lack of understanding of the differences in blending oils for engine and gearbox use, including the all-important additive package.

I have never observed much if any difference in hot or cold oil pressure between 10W-30, 15W-40, and 20W-50 in moderate to high ambient temperature, either cold start or fully warmed up. Some report differences, but they are a minority.

One of the reasons to use a higher viscosity range is engine oil operating temperatures. In hot weather low speed driving, or spirited driving in hot weather, and especially race track hot-lapping, engine oil temperatures can well exceed 100C, and a higher SAE viscosity grade will provide insurance that absolute viscosity will remain high enough to support the bearing load and prevent metal to metal contact. The higher the absolute viscosity, the higher the oil film strength in the bearing, and the more load it will support.

Duke

[Michael H.]

Re: oil viscosity for 67 300 hp in South

Some of you need to go back and look at the Web references in my oil article from the Summer 2008 Corvette Restorer, specifically the engine and gear oil viscosity range comparison chart at 100C:

You can use 15W40 if you want to. I'll stick with the 10W30.

[Larry M.]

Re: oil viscosity for 67 300 hp in South

In an ideal world, 10W-30 is probably the best oil viscosity range for these cars (with factory stock engines). Can use just about anywhere. However 10W-40 or 15W-40 are also good.

Back when these cars were new, many of us (myself and friends) ran straight weight 20W-20 for 9 months of the year, and 30 wt during the summer. Logged tens of thousands of miles without any oil related issues....both city and highway miles. This was in Chicago, where winters got downright chilly and summer temps were 80-100 F.

And grade for grade, the current oils are far superior to these old ones regarding base stock, pour point, sludging, and lubricity. Probably our biggest concerns nowadays are (should be) from dry starts due to minimal use/driving, and ZDDP content.

Larry

[Michael H.]

Re: oil viscosity for 67 300 hp in South

In an ideal world, 10W-30 is probably the best oil viscosity range for these cars (with factory stock engines). Can use just about anywhere. However 10W-40 or 15W-40 are also good.

I agree. People that know MUCH more than we do about motor oil agree too.

My biggest problem with heavy oil is the way most of us drive our cars today. Only a very small percentage drive these things long enough to actually get the oil up to a proper operating temperature before we mash the throttle. It takes at least 15 minutes of driving before oil temp is where it should be and by that time, most joy riders are already pulling the car back into the garage after giving the neighbor a ride, buzzing the engine to 7000 RPM.
The Most important issue, especially with street driven cars that are not "up to normal operating temp, is the wide range in viscosity between 100* oil and oil that is at normal operating temp.
The difference between 30W and 40W may be close at 210* and above but when at 100*, or 150*, the viscosity of 40W is much higher. Too high to properly lubricate and often goes into a condition called sheer. (not to mention the loss of power because of increased drag)
Sheer is when the lubricant can't be moved across the surfaces of the bearing and crankshaft pin. That condition opens a slight gap in the lubricant that allows the metal surfaces to come into contact.

Actually, as RPM increases, the viscosity requirement decreases. (that one oughta get some response)

I MIGHT consider 15W40 if I were going to drive one of these cars cross country in August again but I doubt it. I've done it many times with 30W or 10W30 without issues. So have many thousands of other people in old Corvettes. I'll never use it again for around town driving, which is mostly what we all do.

I've seen "the black death" too many times over the years because of oil that was to heavy. (either because of a poor choice in weight or too cold)

One cool (high 60's) fall day at Road America years ago, three of the guys in our bunch lost engines to the black death (spun main/rod bearings) because they didn't cover the oil cooler and get the oil up to a decent operating temp before running hard.

[Stuart F.]

Re: oil viscosity for 67 300 hp in South

I too used to run 20w-20 practically year around in Wisconsin/Illinois back in the day, and later 10w-30. But, when the concern for ZDDP arose and I'm looking at my "never been apart" 63 L-76, I shopped around for the best source. That turned out to be Sam's club where they had cases upon cases of 15w-40 at a reasonable price - so I bought a few. I've been keeping my eyes open where I go and have yet to see any 10w-30 meeting CJ-I or CJ-4 API specs on the shelf. Granted, I have not looked hard as I have a lot of the Shell Rotella -T in 15w-40 in stock. I suppose some of our less commercial auto parts stores including NAPA may have some. I found a case of Wix spin on oil filters at one place near MCO a number of months back. Perhaps I will check with them to see what they have or can get.

Any other suggestions?

Stu Fox