You were seeing the oil "wick" though the gasket. This can be variable as to how much "thin factions" there are in the particular brand of motor oil you are using. It can be slowed with increased "clamping pressure", but once it starts, there's NO stopping it, unless you change brands of oil.
A friend once cussed Castrol GTX as he couldn't get his valve covers to not seep. I ran that idea by my machine shop operative and he mentioned the "wicking" action. He normally bought the brand of oil that was on sale at the auto supply, rather than being loyal to just one. He said, back then, that he noticed that Valvoline would seep, but when he put Pennzoil in, it dried-up. This was in the later '80s.
Some brands of rubberized cork valve cover gaskets have smaller or larger cork particles and/or more or less rubber in their mix. I usually like the smaller cork bits myself, which usually cost more.
In any event, here's the fix I've used with great success. I'll get some black high-heat silicone and coat EACH side of the valve cover gasket with it. Using my index finger to smear a thin coat of it on. Let it cure and do another side until it's all covered. All FOUR sides and maybe the insides of any bolt holes. This can usually take a day or two to happen.
This effectively seals the gasket from any oil intrusion and wicking. Do NOT attach the covered gasket to the valve cover. Just lay it all in place and put it together. This also makes for a quick clean-up/removal when you take things apart again. I use this technique on ALL gaskets in the engine, paper or rubberized cork.
With gaskets with a bead, as intake gaskets, I'll fill in the bead with the sealer, using a Mr. Gasket gasket scraper to smooth it out. This effectively makes it a "Print-o-seal" style of gasket. Sealing is much better. Still easy to remove and clean up.
In the '60s, we chiseled the 3M "yeller" sealer off of the valve covers, where it was used to glue the gasket to the cover (didn't help it seal any better!) and hammered the bolt holes on the covers themselves for a flat sealing surface. But nothing really helped! I suspect that aluminum valve covers sealed better than the stamped steel ones, but I don't know that for sure.
IF you really need the high-heat tolerance of a solid orange silicone valve cover gasket, then you need that type of gasket. For normal stuff, the "sealed gasket" can work very well.
If you're worried about the fabricated valve cover sealing surface, put a straight edge on the sealing surface and see what it looks like. Or put a thin skin of chassis grease on it and lay it against the cyl head to look for "hills and valleys"? But I suspect that's all in pretty good shape.
With any sheet metal valve cover, be sure to use a "spreader bar" with every hold-down bolt/nut. There were some good ones on the middle '80s Chevy small blocks. Rather than the smaller ones that are more like big flat washers, contoured to fit.
Enjoy!
CBODY67