[snip]
>I had EXACTLY this happen to my '69 bus in Colorado, especially when
>coming to a stop at a stop sign. Turned out to be a mostly-shredded main
>oil seal (yup-- the one around the crankshaft just inside the flywheel,
>that you have to take the engine out to get to).
I was able to replace the rear seal on my '74 without removing the engine. 
Had to remove some of the sheet metal, but it wasn't all that hard. 

-Mark McCulley