Fact: An SAE 20W-50 conventional motor oil flows the same as a SAE 20W-50 Synthetic motor oil. If they are both rated with the SAE spec. then they must flow the same at engine operating temp. Now if we are talking cold temperature flow, synthetics almost always flow better than conventional oils in cold temps.

Fact: Synthetic motor oil does not cause leaks. Leaks can occur any time you change brands or types of motor oils whether it be conventional or synthetic.

Some motor oils can leave behind deposits which can act as a supplemental seal on higher mileage engines. If your main crank seal is bad and you have gunk protecting it from oil deposits, then you probably have no-or-minimal leaks. Now you switch to another brand; it could be conventional or synthetic, both of which have differing levels of detergents and other additives than the oil that you were just using. These differing additives eat away at the gunk that was acting as a supplemental seal and now you have a leak.

The issue was not the oil change, but rather a worn out crank seal.

Fact: There are many synthetic motor oils that have the same or higher levels of Zinc and Phosphorous as Heavy Duty Engine Oil (HDEO).

Example: