I am sure this won't answer your question, but here is what I know. Amsoil is a 100% syn oil, not sure about the Klotz. Petroleum oils tend to shrink rubber and engine seals, which is why they add additives to compensate for this. Synthetic oils tend to swell rubber and engine seals, so they add additives to compensate for the swelling.

The old oil could have thickened up, but if it is a good syn oil it shouldn't have (unless it had some petroleum in it). Pressure buildup due to excessive oil is possible, but unlikely. The Amsoil 10W-40 has a very low volatity rating (less than 5%) so the oils viscosity shouldn't change more than 10% over its useable life.

Tom

Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety. Benjamin Franklin, US author, diplomat, inventor, physicist, politician, & printer (1706 - 1790)