I have had a leak at the oil fill tube since the car
was new. It's the plastic extension tube that twists
into the pass side rocker cover. I have tried RTV
and it still leaks. It also seems like this problem got
slightly worse after I installed the 1.6s. Any ideas?

 

What would it cost for a new filler neck? Maybe try to just replace that?

 

That's what I would do. Or try to add on another rubber groumet.

 

Mine leaked too from where the oil fill neck screwed into the valve cover. I tried RTV a few times...didnt help. Eventually, I replaced my stock valve covers with polished aluminum ones and used the stock Oil cap without the filler tube extension...no more leaks!

 

I think you have the same valve covers as my 1995...I just took off the extension and screwed the oil fill cap directly in...no more leaks...
--Alan