I have been using synthetic oil for over 25 years. 100% synthetics are compatible with conventional petroleum based oil and vice/versa.
There is only one reason engines leak oil after switching to synthetic - the oil seals were already going bad and the synthetic oil just brought it to a speedier result due to the excellent cleaning properties of synthetic oils. Vicosity (thinner) has nothing to do with synthetic oils leaking more - all oils have to meet the same SAE specs. Leaking is directly related to the condition of the engine seals.
The synthetics clean so much better that they will cause a high mileage engine to start leaking. I have recently switched my son's high mileage 4 cylinder engine to synthetic. It does not burn as much oil as it did when we first purchased it - it does not leak any oil.
