Gasket Madness

Early style right crankcase gasket (at top), with the oval oil passage near the head.  And the later style (below) without the oval oil passage.

And just learned from Bill the S90 Guru that the later style gasket is supposed to cover the oil filter opening, and that you're supposed to change the gasket every time you need to clean the filter!

But at least the gasket mystery is finally solved.