No, no, and NO!
Let's keep to the facts
. A 10ga wire is rated for 30 Amps
(and here, in an extension cord, it's terminated with a 15 Amp
receptacle). A 12ga wire is rated for 20 Amps
, a 14ga 15 Amps
, both have the same 15A receptacle at the end (the "vertical only" 2 blade, with the Neutral wider than the Hot).
Your Block Heater draws less
than 15 Amps
(it's only 1,000 watts - your wifes Hair Dryer, your Toaster Oven, Coffee Maker, all are usually 1,500 watts).
So, 10ga? Absolutely
unnecessary, I mean, couldn't hurt,
but to imply you need something like that is just wrong. I think the only reason not to use 14ga is not because of it's lack of capacity, but rather you could have to run a long length to get to your truck (over 50ft), plus they tend to be cheap & flimsy for something that's getting "dragged around" and tugged on.