I believe you shall be fine! I believe they may state not to use silicone because during tightening it can deform real easy and squeeze out causing a leak. Silicone on rubber is very slippery. But, once it cures and you were successful, the bond is great. Go back and check the alignment of the gasket around the outside perimeter. If you can see the gasket everywhere or know its there from the silicone, then I'd certainly run it. From what I see, it looks great and will not leak!
Its sort of like the lower rubber seals they supply with intake manifolds. When you add silicone sealer, they squeeze in every direction, and sometimes the seal is lost and then they leak. But, if successful, it works great. Fortunately with the oil pan, there are bolt that go through the gasket every 2 1/2 inches which keeps it fairly straight unlike intake manifold seals which are not held in place by any edge or through-bolt.
Good Luck! Thank your Gramps because he has probably seen a lot of leaks and knows silicone is good stuff. And, that black RTV is good stuff other than the extreme mess it makes with what it touches ... also believe its for high temperature also.
