and because you say it is doesn't mean it is.
at least I have 2 examples to look at.. one with syn oil in it it's entire life... that 450 has had syn oil since the day after it rolled out of the factory after the stuff it came with was drained out.
too many people blame and attribute absurd things to situations without sufficient evidence to carry the argument.
Back in the middle ages they thought tomatoes were poison and that frogs were born out of mud and flies came from rotting meat. they say some limited but incomplete cause and effect and missed a few important issues inthe middle before drawing a conclusion. Oh yeah.. earth ain't flat either.. not anymore... :)
I'd think a new engine.. or a rebuilt engine would be a good candidate for a syn switch.. vs an aged one.
no way to post diagnose your rig now.. but i'd blame the rear seal or the install or the retainer or the sealing surface before I'd balme the oil.
I've seen many replaced mains on tractors only to see an annoying drip... oil would be on the last of a list for what I would think the cause was attributed to.