I don't really know oil viscousity too well. I know a 10W30 is thinner then a 10W40 but I am not sure how to makes sense of 20w50 oil.

Guy I am buying a 74 Formula 400 off of uses this stuff. It has had the oil changed every 3K since brand new.

thanks!

 

When talking about 5w30, 10w40 or 20w50, the "5", "10" and "20" refer to the oils' viscosity at cold temperatures (can't remember, but lets say around the freezing point, just for argument's sake) and the "30", "40" and "50" refer to the oils' viscosity at high temperatures (usually around typical engine operating temperature). If you compare a 5w30 to 10w30 oil, that just means that the 5w30 will pour more easily in cold temperatures (it's less viscous).

SO, to answer your question, 20w50 is even thicker than 10w40.

 

thanks. Maybe when I get the car I should switch to a lighter weight oil. Like...10W30?

 

Depends on the season and your area's climate. You have to consider the coldest starting temp of the engine when picking a variable weight oil. I have a chart in my Harley service manual that recommends the different types of oil vs. temps. I could look it up for you when I get home from work (if I remember). As far as I can recall, 50w oil is good as long as the temp doesn't get below 60 degrees, so if he's been running it in colder temps, he could be getting some dry/hard starts.

 

This also depends on how the engine was built. Many racing engine are built with greater than normal bearing clearences and therefore require a "thicker" oil. 20W50 was practically the standard racing oil throughout the 60's and 70's. However there are very few time when a stock engine needs a W50 weight oil. The prevailing trend today is to use a lighter synthetic oil that gives better protection with less friction and lasts longer to boot.

 

Well the engine has been rebuilt but just to stock specs. When I get the car it will not be driven in winter and just spring/summer/fall once or twice a week.

 

In my irregular use vehicles, I use this product http://www.amsoil.com/products/ame.html . It's rated for gasoline engines (the SL rating), and has extra corrosion protection additives vs. normal engine oils. The 15W will be fine in summer (it's been fine through the winter for me as well - nice low pour point), and the 40 viscosity is what was regularly used in the early 70's, anyway.

 

As said above, unless an engine has bigger clearances, either built that way for racing or from wear, it will run better with thinner oil. 50 will give better pressure, but it doesn't flow as well. Bearings need a good flow of oil to carry off the heat.

 

Let's take 5w30 oil for example: The 5 is the wt of the oil. the 30 is additives added to the oil to give it 30 wt properties. Depending on Ambient temp and barometric pressure the additives shrink for colder temp and expand for higher temp creating the illusion of a thicker oil. And from shrinking and expanding over a certain amount of time they loose there ability to change. That's why you need to change your oil often because eventually it becomes just 5wt oil. now a straight wt oil doesn't change, it stays the wt no matter what temp and baro pressure.

 

While what you said is generally true, it is an over-simplification.

The "W" viscosity and "last number" viscosity are tested under the different temperature conditions (as stated earlier). Straight-weights are only tested at the higher temp. It is true that "viscosity improver" additives are typically used to produce "multi-viscosity" or "all-season" oil; the VI improvers are long-chain polymers that "thicken" the oil at the higher temperatures by expanding and taking up space between the oil molecules so it will meet the "last number" viscosity requirement. In days of yore, I've seen Penzoil 20W-20, which just means it meets the viscosity requirements for a 20 weight oil at both the low and high temps. However, it is possible to meet both the "W" and "straight" viscosity requirements w/o VI improvers - typically only accomplished by synthetic-based oils.

The problem with VI improvers doesn't come with repeated heat cycles, but that they shear under load and oxidize with heat. They don't do their job when either of those occur.

Petroleum-based oils, even straight-weights, are also more prone to oxidation than synthetics. That is the primary reason you have to change your oil. Straight weights will change viscosity with oxidation and evaporation of the lighter molecules.

I'll have to admit I've never seen any correlation between barometric pressure and oil life.

But, back to the originator's situation: a 74 Pontiac 400 rebuilt to factory specs driven primarily in warm weather would be fine with a 10W-30, 10W-40, 15W-40, etc., etc., etc. Since cold weather starting isn't an issue, there isn't any particular need to go with a 5W-anything. There also isn't any need to go to a 50 weight. I stand by my recommendation above based on the intermittent use expectation (great wear protection, too).

 

I didn't say baro had to do with oil life. It has to do with the oil knowing what climate it's in.

 

While labs need to control that while doing standardized testing, your engine breathing is going to be affected a lot more by changes in barometric pressure than the oil is. As far as I know, piston engine aircraft don't even call out barometric pressure as an oil operating parameter (but, I'm a lot more familiar with automotive applications than I am with aircraft...).

Ambient temperature is an issue, however.

 

thanks guys. I will likely change it to 10W30.

Now--should I go regular, semi-synthetic or full synthetic?

 

Well, how much protection does your engine deserve? $1.50/quart regular? $3.00 semi? $5.00 synthetic?

What's an engine cost?

 

I run Royal Purple 20/50 year around