[TR] Timing case oil leak

[Stuart Greenwood]

In July 2017 I saw that the front crankshaft oil seal was leaking on my TR250 but the timing case itself seemed to be leak free. In January 2018 I replaced this seal which of course meant removing the timing case cover and using a new gasket. I serviced the car last weekend and could see that the oil seal was not leaking but there seems to be oil leaking through the timing case gasket. 
I have the same situation on my Spitfire Mk IV. I completely rebuilt the engine in April 2004. Now, after about 15,000 miles since the rebuild  there are signs that the crankshaft oil seal is leaking again  and the timing cover gasket is leaking oil. 
I am starting to think that the gaskets you can buy are not oil leak proof. I think that over time oil seeps through the gasket material, NOT between the gasket and the engine block or between the gasket and the timing case cover but through the gasket material itself.
I am going to replace the oil seal on the Spitfire but the question I have for the list is should I use a gasket or use "form a gasket"  RTV type stuff? Have any of you used a gasket that is guaranteed not permeable to oil and if so where did you buy it from?

Has anyone else suspected that some gasket you can by from reputable dealers like TRF, Moss or Rimmers are in reality permeable to oil?

Stuart A Greenwood
San Diego Ca