it will be a BTA12-600"B" not "8"

    They are rated at 12A 600V (see what the markings mean now - "12" for 12 Amp and "600" for 600 Volts)

    The original BT138-500E is rated at 12 Amp 500 Volt

    So the one you have is a superior device and the pinout is identical to the original - it will drop straight in.

    The heatsink transfer compound can be pretty well anything - the stuff used for PC heatsinks is fine.

    .