SYNTHETIC OIL IS BETTER
Use synthetic lubricant when you change your engine oil. It's more expensive, but your engine will thank you.
Synthetic oils last longer, stand up to high heat and extreme conditions, and have better flow rates when cold than regular oils.
Take care, however, not to void your warranty.
Follow the schedule for oil changes specified in your manual – at least through the warranty period.
HEAVIER IS NOT ALWAYS BETTER
Use the oil viscosity grade that's recommended in your owner's manual for the temperature range you expect for the coming season.
Lighter grades (lower viscosity, such as SAE 5W-30), often specified for today's smaller car engines, will deliver easier starts and better engine protection in winter and improved gas mileage throughout the year, thanks to less internal engine friction.
Do not use a heavy grade of oil in cold winter climates or you will risk damage to your engine.