What oil

Synthetic vs. conventional oil per the world wide webs:

Synthetic oil is better for your engine than conventional oil. Although conventional oil (i.e., mineral oil) can provide adequate lubrication performance, it can’t compete with the overall engine performance and protection provided by synthetics. Synthetics use higher quality base oils as compared to the less-refined base oils used in conventional oils which makes conventional oils:

Less chemically stable
Oxidize and acidify more easily
Quicker to break down and lose protective qualities

Overall, full synthetic oils provide better engine performance and protection than conventional and synthetic blend motor oils. Full synthetic motor oils are specially formulated with a tailored additive system that provides additional performance benefits above and beyond those found with conventional and synthetic blend oils by:

Combating sludge and deposit buildup
Reducing wear
Protecting against high temperatures
Controlling oil breakdown

It was once explained to me that in addition to having less impurities than conventional oil, synthetic oil has molecules that are all basically the same size. Those molecules are like ball bearings in this application. Conventional oil has a variety of different size molecules that don't interact as easily with each other as the synthetic ones do. There are no negatives to using synthetic oil.