If oil is too thin
A lubricant that is too thin for the operating condition may not maintain sufficient film thickness between moving surfaces, increasing metal contact and wear risk.
If oil is too thick
Excessive viscosity can slow flow at start-up and increase frictional drag. Engine-maker viscosity charts account for ambient temperature and engine design.
Specification first
SAE 15W-40 or 10W-30 identifies viscosity behavior, not the entire performance requirement. API category and OEM approvals/specifications must also match the engine.
This article is general technical information and is not a substitute for the equipment OEM manual, lubricant manufacturer recommendation or site-specific engineering review.