It's not a matter of washing head to toe, it's a matter of scrubbing thoroughly enough everywhere and not missing a spot.

I assume you're referring to fels naptha bar soap? Despite the name, it doesn't actually have naphtha in it any longer.

https://en.wikipedia.org/wiki/Fels-Naptha

Even worse, it is almost impossible to find these days. The brand was acquired and the company that bought it is still ramping up production and distribution. Source: I emailed the company.

Actually just bought some Zote (also now owned by the same company), just yesterday, to test it out... but from what I hear, I don't think it is as good.

My grandparents always swear by using "brown soap" if you think you've been exposed to poison ivy. aka Fels-Naptha. It's a pretty strong soap, but I don't think it contains naphtha anymore.