Rick's Free Auto Repair Advice

Why you shouldn’t change to a different motor oil viscosity

Here’s why you shouldn’t second guess the engineers and change to a different motor oil viscosity

If you change motor oil viscosity to one that’s not recommended by the carmaker, you cause serious problems like premature wear, a check engine light, and even camshaft damage.

Read this article to find out what motor oil viscosity you should use.

How carmakers arrive at the recommended viscosity

The recommended motor oil viscosity is based on the following factors.

• engine bearing clearances
• camshaft design
• whether the engine uses a variable valve timing mechanism
• the timing chain and tensioner design
• the type of cam followers used in a GDI engine
• proper turbocharger cooling
• startup in cold weather (pumpability and flow)
• fuel economy

What can happen if you change motor oil to a different viscosity

If you change to a thicker (heavier weight) oil, you will experience

• Lower MPG due to higher internal friction
• Harder starting in cold weather (if you raise the W viscosity)
• More engine and turbocharger wear due to the oil’s higher internal heat generation, along with its lowered ability to remove heat from friction areas due to lower flow rates.
• Variable valve timing trouble codes due to a change in commanded versus actual advance/retard changes

If you change to a thinner (lighter weight) oil, you will experience

• Lower oil pressure at idle
• Variable valve timing trouble codes due to a change in commanded versus actual advance/retard changes

How oil viscosity affects variable valve timing

Variable valve timing alters the camshaft timing by rotating the cam in relation to the

image of variable valve timing solenoid

Subaru Variable Valve Timing Solenoid

cam belt or timing chain. The rotation is executed using pulsed oil pressure.

In order to command a cam timing change and then check the results, the PCM assumes you’ve used the proper oil viscosity.

Switching to a higher or lower viscosity oil will change the performance of the camshaft VVT solenoid or phaser, so the computer won’t see the cam timing result it expects. That’ll set a check engine light and trouble code and will affect engine performance. Improper cam timing can result in many different performance issues, from rough idle to lack of power.

In other words, when a car maker recommends a certain type and viscosity, the PCM is programmed to make cam timing changes based on that viscosity. When you use a different oil, you get different results, and they’re not what the PCM expects to see.

How oil viscosity affects GDI

oil life with synthetic

High-pressure fuel pump and camshaft lobe wear

GDI vehicles have two fuel pumps; an electric pump in the gas tank and a second pump on the engine that runs off the camshaft.

It’s the 2nd fuel pump that is the biggest concern when you change motor oil viscosity.

The 3-lobed cam used to drive the high-pressure fuel pump demands the exact oil recommended by the manufacturer.

Substituting a different oil can cause catastrophic damage to the camshaft that drives the high-pressure pump.

Shops are seeing  GDI vehicles where the lobes are completely worn off the cam due to the wrong oil.

Car makers have made it known that they will NOT pay for warranty claims based on improper oil viscosity. Screw with the type of oil or change the viscosity and you can mess up your variable valve time devices, get the wrong tension on your timing belt or timing chain (causing jumped teeth or total engine destruction in an interference engine).

Moving to a higher viscosity oil can cause hydraulic lifters to literally “pump up” and bend push rods on a cold start. Keep in mind that in car talk, a cold engine is one that hasn’t run for three or more hours. You don’t have to live in a cold climate to be affected by these kinds of problems. Oil simply doesn’t flow well when it’s cold. So follow the car makers’ viscosity recommendations and quit trying to second guess their specifications.

© 2012 Rick Muscoplat

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Posted on by Rick Muscoplat



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