How an you tell if the engine oil you are running has the correct viscosity?
I used to run 10W30 in a freshly rebuilt smallblock. One of the threads on this forum (can't find it now) discussed the idea that too heavy an oil just increases the amount of time for the oil to get everywhere on a cold startup. Made sense to me at the time.
So, at the last oil change, I switched to 5W30, and rightaway noticed the amount of time where the cold oil pressure is pinning the (80psi) gauge is very much less than before. Hot oil pressure remains about the same- 55 to 60 psig.
Maybe I'm imagining it, but it seems that now the engine is a bit noisier. When I pull into the garage, and the engine sound is reflected right back at me from the garage walls, it sounds slightly different.
Is the 5W30 too thin for my engine?

 

No, 5W30 is NOT too thin for your engine. I run 5W30 in my 800ish HP, 540ci BBC motor. 5W30 and 10W30 are the same basic viscosity at operating temperature. So, unless you are running entirely different types of oil (conventional vs synthetic), any difference in sound should be perception more than reality. If they were the same brand and type of oil, with their only difference being the first number cold viscosity rating, then their only real difference is at cold start-up. See below for more details.

THE BENEFITS OF USING THINNER OIL ARE:

• Thinner oil flows quicker at cold start-up to begin lubricating critical engine components much more quickly than thicker oil can. Most engine wear takes place during cold start-up before oil flow can reach all the components. So, quicker flowing thinner oil will help reduce start-up engine wear, which is actually reducing wear overall.

• Thinner oil also flows more at normal operating temperatures. And oil FLOW is lubrication, but oil pressure is NOT lubrication. Oil pressure is only a measurement of resistance to flow. Running thicker oil just to up the oil pressure is the wrong thing to do, because that only reduces oil flow/lubrication. Oil pressure in and of itself, is NOT what we are after.

• The old rule of thumb that we should have at least 10 psi for every 1,000 rpm is perfectly fine. Running thicker oil to achieve more pressure than that, will simply reduce oil flow for no good reason. It is best to run the thinnest oil we can, that will still maintain at least the rule of thumb oil pressure. And one of the benefits of running a high volume oil pump, is that it will allow us to enjoy all the benefits of running thinner oil, while still maintaining enough oil pressure. A high volume oil pump/thinner oil combo is preferred over running a standard volume oil pump/thicker oil combo. Because oil “flow” is our goal for ideal oiling, NOT simply high oil pressure.

• Oil flow is what carries heat away from internal engine components. Those engine components are DIRECTLY oil cooled, but only INdirectly water cooled. And better flowing thinner oil will keep critical engine components cooler because it carries heat away faster. If you run thicker oil than needed, you will be driving up engine component temps.

• Thinner oil will typically increase HP because of less viscous drag, compared to thicker oils. That is why very serious Race efforts will generally use watery thin oils in their engines. But, an exception to this increase in HP would be in high rpm hydraulic lifter engines, where thinner oil can allow the lifters to bleed-off at higher rpm. And in everyday street vehicles, where fuel consumption is a consideration, thinner oils will also typically increase fuel economy. The majority of new cars sold in the U.S. now call for 5W20 specifically for increased fuel economy. And now Diesel trucks are increasingly calling for 5W30, also for fuel economy improvement.

• With the exception of high rpm hydraulic lifter engines, almost no engine should ever need to run oil thicker than a multi-viscosity 30 weight. The lower the first number cold viscosity rating, the better the cold flow. For example, 0W30 flows WAY better cold than 20W50. And 0W30 flows WAY better cold than straight 30wt, which is horrible for cold start-up flow and should be avoided at all cost. And the lower the second number hot viscosity rating, the better the hot flow. For example, 0W30 flows WAY better hot than 20W50.

• Thicker oil DOES NOT automatically provide better wear protection than thinner oils. Extensive “dynamic wear testing under load” of dozens and dozens of motor oils, has shown that the base oil and its additive package “as a whole”, is what determines an oil’s wear protection capability, NOT its viscosity. For example, some 5W20 oils have proven to provide OUTSTANDING wear protection, while some 15W50 oils have only been able to provide MODEST wear protection. So, do not run thicker oil under the false assumption that it can provide better wear protection for our engines.

• BOTTOM LINE: Thinner oils are better for most engine lubrication needs.

540 RAT
Member SAE (Society of Automotive Engineers)