DwaliX,
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.