I have a factory Isuzu block heater, but since I just moved, I can't find it or the instructions that came with it...
But what I can tell you, is that it's threaded, & sealed with an O-ring.
A call to Merlin may help shed some light on this for you.
I know that you're looking for the heater warmth to come on sooner in the morning, but FWIW, mine has always started without much hesitation in temps as low as -35.
IMO, synthetic oil & an Optima red top help with that...
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