My boat is wired with THHN/THWN in FS boxes, conduit bodies and RNC. 35 years in salt water hasn't fazed it. I even have some yellow wirenuts in there (EEEK!!). They do have a squirt of silicone grease in there. The only warning I would have is "flexible" doesn't mean the same thing in a car or boat. If this wire is just going to lay there, well restrained it will outlast that auto parts store wire. If it's going to move every time you hit a bump, buy the good stuff.
