A reader asks: I’ve got a Volvo-Penta marine engine. The recommended engine oil is straight SAE 30. What do you suggest?

AMSOIL 4-Stroke Marine Synthetic 10W-30 Engine OilOur answer: AMSOIL Synthetic 10W-30 Marine Engine Oil would be at the top of our list. The “straight-weight” SAE 30 recommendation is derived from antiquated thinking on motor oil. Some conventional multi-viscosity motor oils could be prone to shearing and thinning when exposed to extended periods of heavy loads. The AMSOIL Marine 10W-30 is highly resistant to shear down, so that simply isn’t a factor. AMSOIL Marine 10W-30 would meet the viscosity requirements of your engine through the entire service interval.

It is important to note that AMSOIL Synthetic 10W-30 Marine Engine Oil contains very powerful anti-rust/corrosion ingredients for marine use. This product was engineered to stand up to all of the rigors of marine use.

Why Is Shearing An Issue With Some Motor Oil Formulas?

One of the main reasons that some multi-grade motor oils are highly prone to shearing is that they may contain “viscosity index improvers”. “VI” improvers expand with heat to prevent the motor oil from thinning out as engine temperatures rise. Imagine tiny springs that expand when exposed to heat. The problem with VI improvers is that eventually they can shear down reducing the overall viscosity of the motor oil. This compromises the oil’s ability to fight wear in extreme heat and contributes to depositing. Some conventional motor oils can contain up to 20 percent VI improvers by volume. They offer no benefits in terms of lubrication.

AMSOIL motor oils do not contain VI improvers. The synthetic chemistry of AMSOIL Marine 10W-30 makes it highly resistant to heat and shearing. That’s why it is the perfect candidate for Volvo-Penta engines that suggest an SAE 30 motor oil.

We do have a motor oil that could be consider a straight SAE 30. AMSOIL SAE 30/10W-30 Synthetic Heavy-Duty Diesel Oil is suitable for diesel and gasoline engines. We would still lean toward AMSOIL Marine 10W-30 because of the obvious benefits of the marine anti-rust/corrosion package.