I think that back in the late 50s early 60s that Shell and Texaco straight wt non detergant oils leaked less--the multi grades leaked so bad out of the GM Corvair aircooled engines that they started telling everybody to use Shell/Texaco.
The new modern syn oils of 0-x wts will be pretty much impossible to be 100% leakproof in a
oil system that isn't a full vacume assisted crankcase