Leak on oil strainer flange

[Streetcar]

I have tried two different types of gasket material on the oil strainer flange on the sump of a small diesel engine. It still leaks oil.

The flange has two bolts which screw into the side of the oil sump. The manufacturer supplied cork gasket disintegrated straight away and leaked oil from the flange.

I then cut my own paper gasket using thick paper (1mm) gasket material. This was better, didn't disintegrate, but still leaked small amounts of oil. Even after re-tightening the bolts.

When I took it off again there was no apparent leak paths through the surface of the gasket material. Any ideas why? Could the paper be too porous?

I will try again. I have found some much thicker, stronger cork gasket material. Would this be better than thicker paper material?