Which primer for enamel paint

I have some Bill Hirsch engine enamel I'm using on my engine. The directions say that on cast iron parts, the paint can go on directly without primer. But, when painting smooth surfaces like oil pan and valve covers, that it's a good idea to use a primer.

I have some of their high temp silicone based primer for exhaust. I called their customer support and they said it can be used as primer for their engine enamel.

Now, I've never used a silicone base for an enamel paint, and the guy that I talked to on the phone didn't exactly sound enthusiastic about his suggestion, so I'm a little nervous about it.

I have some vht engine enamel primer as well... which primer would you use? The engine enamel primer or the hi temp silicone primer? If something else, what?