DC-4 is Dow Corning #4, as Plummit explained. Aircraft Spruce sells it for $18 (plus shipping).
As far as using DC-4 vs oil on lubricating the oil filter gasket -
I'm an old farm boy, and all I ever used on my ag equipment spin-on filter gaskets was oil. I too, never had a problem with the filter leaking or sticking using oil. However, on the side the Champion aviation oil filter I use, it states to use (first choice) DC-4 to lube the gasket. Oil is the second choice.
Lycoming also recommends DC-4 on engines having the 15047 oil filter adapter
So, this is why I use DC-4 on my oil filter gasket. But, As Gasman stated - he's been using oil for years - so don't worry.
Yes - I cut my filters open and inspect. EAA has a video on their website on how to do it. Below is a link to a pdf from Blackstone on filter inspection.
Hope this info is helpful