yep they can get old and brittle or fall in or off over time esp. when you work on them. he may have not changed them, depends on the engine, most long blocks don't have intakes or valve covers. I would use some gunk engine degreaser on everything ecept like on top of the intake since the tbi or carb(87 starteed tbi in trucks with small blocks as best I know) then sensors and distributor, don't want them wet really. I would do the block and valve covers and frame, fenders etc. Then you can follow up with like carb or brake cleaner on a rag, but be warned it takes paint off like paint thinner.
Once it's all clean and free of oil drive it some and see where it comes from.
That or look for a wet spot before doing anything, a fresh leak will be wet or shiney looking.