Are all subaru classic head gaskets the steel type and do they vary in thickness throughout the years and various versions?

 

The Multi Layer Steel (MLS) head gaskets were used on the Version 3+4 STi engines and are 1.60mm thick.
Composite Gaskets were used on Version 1 ~ 4 WRX.
Version 5 onwards STi and WRX use MLS gaskets and are 0.90mm thick IIRC.

Mick

 

Merlin, according to RCM's site the MY99/00 ones are 0.6mm thick.

So OEM gaskets are...

MY93-96 - 1.6mm fibre composite
MY97/98 - 1.6mmm MLS

MY99/00 - 0.6mm MLS

But you can upgrade your MY96's fibre gaskets to the MY97/98 MLS ones, which are stronger.