[Dave in MN]

I don't know if RTV would bond well enough to keep this area sealed. I have used Permatex "Right Stuff" as a gasket replacement and find it takes a stiffer set and bonds very well to cast iron if it is clean. IMO: I would use this product instead of the RTV. Try to work some of this same product along the seam between the cap and the block to halt any oil movement. I think it may solve your problem. Let the Right Stuff set up 36 hours before running the engine and try to keep the Stuff no less than 1/16" thick over the area to be sealed and beyond to help support and bond the fix.
Good Day!