Can anyone tell me how often do you need to change the front Gasket in a 90 300se the one where the fan is mounting to.

Do you mean the horseshoe seal between the timing chain cover and the head or the gasket between the timing chain cover and the block? In either case, you change it as often as it bothers you. Mine has been leaking for over two years and I continue to ignore it. My apologies for the stupid sounding answer, but there is no prescribed time interval for replacing these items.

Sixto
91 300SE
87 300SDL