The single most important property of a lubricant is the viscosity of the oil used as a barrier between two meeting surfaces. Viscosity is reported in ISO viscosity grade. IOS 68 has slightly more viscosity than ISO 50.
The equipment manufacturer selects the lubricant viscosity based on the design conditions.
Viscosity is a measure of the fluid’s resistance to flow. If a machine is heavily loaded then the machine designer will use a lubricant that resists being pushed around, which would be heavy like honey. If the machine runs very fast then the machine designer will specify a lubricant that can get out of the way, and back into the way just as quickly. Generally, machines will have either one or the other to be concerned about; sometimes both at the same time.
Your question provides little useful information to aid other in providing a more useful answer. If your concern is based on an operating temperature problem, be sure to get very good, well-informed advice. It is not unusual to find that switching to a more viscous oil because of high operating temperatures actually causes more severe temperature problems because of the fluid friction of the more viscous oil.
The viscosity of the oil is a major factor, but additives or different basic oil composition should all be given due consideration.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
