The starter will draw the most amps to get moving. Once cranking it will hold a steady amp draw under 150.
What do you think 6+ gallons of 15w40 does in cold temperatures that could impact the starter in terms of ability to turn vis a vis amp draw?
You cannot plan everything for best condition scenarios....
We shouldn't be adding variables to this, we should be out enjoying and or working on our trucks.
Safety first. As an emergency vehicle upfitter (installing lights/sirens/computers/communication,etc), and having to fix other people' screwups, I can tell you vehicles DO catch fire because of poor wiring and substandard equipment...Most of the time it's fairly minor, but I have seen vehicles go up in flames, $15k in equipment and $25k vehicle, complete loss, because of an innocent mistake (officer tried to wire in new equipt by himself).
The biggest part of this hobby/community, IMO, is the sharing of knowledge, that includes having the cajones to step up to someone and say 'uhhh, hey, that doesn't look right, you might wanna reevaluate that'... Ultimately, it's their rig, but knowing what I know, I wouldn't suggest one like that, it took me a couple DAYS to finally decide on the proper cutoff switch... Just saying... And if a guy who was an expert on the *insert system here* and tells me my modification of *system* needs to be double checked, and shows why, I'd definitely double check it, even if I thought I was still right.
Batteries are one of those things YOU DO NOT WANT TO BE WRONG ON......Shorted batteries, melted contacts, etc...
and about the cutoff, you do realize you're supposed to switch the ground right, and not the positive? If the switch is damaged or fails, or a wire comes loose, and contacts metal, no harm no foul. If the + wire contacts grounded metal, you can explode the battery...