You don't want to use HEET additive on a diesel I believe. You'd need like Diesel 911 or PowerService or another diesel-specific additive.
I've never had much luck with the anti-gel additives actually working, I've always had to wait for it to warm back up.
Since you haven't changed the filter recently, it's probably worth doing. But if the filter head itself is leaking, that won't fix the dying problem. You can get rebuild kits for the filter head, but you can also get a brand new one for not much more money, so that is what I usually do now. I find the new ones last longer than the rebuild kits.
But don't just throw parts at it, figure out what it needs first.
If you don't KNOW you have winter diesel in, then it could just be gelled up. Wait for the weather to warm up or get the truck inside to warm up, and see if it works again. If so, get winter diesel in it and you're done. If not, test for an air leak and go from there.
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