O-rings are made to be elastic as they are usually stretched to fit into a groove before being compressed. You need to be careful of how you stretch the o-ring while installing it. In fact, you should stretch it more than 50%, and the stretching should be on all the sides, rather than one side.
Lay out the pieces of the new gasket on clean newspaper, and note how the joints fit.
Have you found the right oil seal for your application? The next step is fitting the oil seal correctly, so that it remains undamaged.
Oil seals are always exposed to a lot of chemicals, both mild and harsh chemicals. The seals react by showing some signs like cracks, blisters, and discoloration especially when the chemical is harsh. This clearly shows that the chemical is not compatible with the seal, which goes as far as affecting its cross-link density (increase or decrease). When the cross-link density increases, the seal material becomes harder, but when it decreases, the seal material becomes softer.