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.
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