40W thicker than 20W-50

The caller with the cooked engine should burn (er, use) a thicker oil, but I think “20W-50” means 20W viscosity with additives that give as good lubrication as if it were 50W. But that doesn’t mean it it has viscosity of 50W, just lubes as well as 50W.

So I think he’d be better off running 40W rather than 20W-50.

It means that when the engine is cold the oil behaves like a 20W oil and when its hot it behaves like a 50W oil.

The engine is toast anyway, but if I was to try to limp it along I’d probably go with something like a 20-50 along with one of those mechanic in a bottle things. I’d rather not have to keep starting the engine up from cold on 40W oil.

Right, all modern cars are built to use multi-weight oils. Single-weights that meet carmakers specifications are non-existant, and should not be used.

What does “behaves like” mean? What I have heard/read is the lubrication qualities of 20W-50 are like 50W when hot – the film strength is like 60W, but the viscosity is more like 30W. Maybe I heard wrong, I was unable to find any suitable graphs/tables in the intertuber-web-net.

C&C said the engine was toast anyway and the guy should not worry about cold starts – “use as thick an oil as possible”. Most drivers are interested in engine protection, but he’s not interested in engine protection, he is interested in oil loss. 40W that meets car specs is still widely available. 50W is not, so if 20W-50 is thicker at hot, that’s the better choice; if 40W is thicker at hot, that’s the better choice.

I agree: stick with a car-spec oil. Pouring 50W gear oil in the engine is a bad idea.

At operating temperature (approx 210 F) a 20W-50 has the viscosity of a 50W oil at the same temp. But when the engine is cold it has the viscosity of a 20W oil.

This might be helpful: http://www.upmpg.com/tech_articles/motoroil_viscosity/

Chris T.

I think “20W-50” means 20W viscosity with additives that give as good lubrication as if it were 50W. But that doesn’t mean it it has viscosity of 50W, just lubes as well as 50W.

The two viscosity values, 20W and 50, are obtained by different tests and therefore are not comparable. The cold value of viscosity, 20W in this case, is obtained by rotating a motor-driven horizontal paddle-wheel in a cup of oil at some specified temperature of 0ºC or below, and measuring the motor current required to obtain some specified speed. The oil is assigned an arbitrary cold viscosity number dependent upon the paddle-wheel motor current.

In petroleum-based motor oil, the cold-viscosity value is set by the addition of pour-point depressant additives, which prevent the wax molecules in petroleum-based oil from crystalizing and freezing together into a solid mass. The cold viscosity number only has operational significance at oil temperatures below freezing. In warm climates for example, except perhaps for a few cold days in winter, there is no difference between 0W20 and 10W20.

The operating viscosity number, 50 in your example, is determined by the time required for a given quantity of oil to flow through a specified orifice. The test is always conducted at 100ºC. In both petroleum and synthetic oils, the operating viscosity is raised by adding viscosity index improvers. At low temperatures these are long-string hydrocarbons that have little effect on viscosity. At higher temperatures the viscosity index improver molecules curl into a coil-like shape and make the viscosity greater than what would occur without them. The basic difference between a 5W20 and a 5W40 oil are the viscosity index improvers. Otherwise they may be made from the same base oil stock.

The viscosity index improver molecules have a tendency to shear (i.e., break apart) in situations of high temperatures and rpms (as in tubos). This causes the oil to degrade into a lower viscosity oil, so that a 5W40 might degrade to a 5W20 oil. For this reason high viscosity oils, when used in extreme conditions, must be replaced more often than lower viscosity oils.

I would be tempted to run straight 40 weight in this engine that is on its way out. I have seen two lawnmower engines that spun connecting rod bearings when 10W-30 was used instead of straight 30 weight as recommended in the manual Apparently the 10W-30 does not behave as a true straight 30W under the extreme heat of an air cooled engine. I have a lawnmower that I purchased in 1988 and have always used straight 30 weight and the engine is still going strong.

I had a 1947 Pontiac with a well-worn engine. The engine used about a quart every 200 miles. I used 30 weight in the winter and 40 weight in the summer to keep oil consumption under control.

Large diesel engines in trains, boats, etc., use straight 40 oil. However, the engines are prewarmed, as is the oil before startup in cold weather. The whole reason to have multi-grade oils is to have a low enough viscosity at startup so as not to damge the engine.

In tropical countires 20W50 is a very popular oil for both cars and trucks.

Synthetics allow a 0W40 or 5W50 for maximum protection in all weather.

The callers wanted to get one more month out of the engine that had already been run low on oil. In this case, cold weather protection and start up wear isn’t a problem. The vehicle, an old Dodge van is to transport the musicians and their instruments over the highway to different gigs. I don’t think at the rate this engine burns oil that synthetic oil would be cost effective.
I was riding with a classmate with some other students in his ancient 1950 Pontiac. The engine started knocking badly and we knew that a rod bearing was about to go. When we were 25 miles from campus, we pulled into a 24 hour service station. The mechanic filled the crankcase with 90 weight gear lube. We made the last 25 miles back to campus. The next day, the wrecker arrived and towed the Pontiac to its final resting place.

I think “20W-50” means 20W viscosity with additives that give as good lubrication as if it were 50W. But that doesn’t mean it it has viscosity of 50W

You’re right about the first part, additives are used to improve the properties of a 20W oil to make behave like a 50. But you’re wrong on the second count, the modifiers change the viscosity to make it have the effective viscosity of a 50. On the other hand if you use a synthetic oil there are no modifiers, the oil is made to have the said viscosity properties.

Straight 50 weight oil that’s hot is actually thinner than cold 20W oil. 20W-50 does not get thicker when it’s hot, it simply does not thin down as much as straight 20 weight oil would when it gets hot.

20W-50, 10W-40, etc. are not viscosities, they are viscosity indexes. The actual viscosity of an oil depends on how hot it is and is measured in SUS (Saybolt Universal Seconds) or centipoises (a metric unit of viscosity).

A lot of modern lubes have a multi viscosity index because the rate of thinning with increase of temperature curve does not match the curve of a single viscosity index.

The viscosity of 50 weight oil is higher than 20 weight oil at all temperatures.

And the viscosity of an operating temperature (210F or so) 20W-50 is higher than the viscosity of a straight 40 weight at the same temp.

Thanks very much for the explanations. I think what I must have heard/read was about breakdown making it runny when run too hot too long, and confused that with being runny when hot. Thanks for clarifying, everybody!

And sorry for calling it “40W”, should have been “40 weight”.

I’d still love to see a graph if somebody finds one.

Here’s a link that compares viscosity of 10W, 30 weight, and 10W30 with a graph.

The above, direct:

Looking at the above graph, notice that the multigrade oil does not increase its viscosity as it gets hot, it just doesn’t thin out as much as single weight oil does. It’s SAE viscosity index increases but its absolute viscosity actually decreases.

The graph clearly shows that cold 10W has a higher viscosity than hot 30 weight oil does.