Parts dept says my rear turn signal takes a 3057 bulb, so I buy it but when I detach the socket I find a 4057 was installed at the factory. Kid at the parts counter says they are equivalent.

Is this true?

 

They are interchangeable and will not create any problems. There is a 2 or so, watt difference between the 3057 and the 4057. The 4057 has the higher output.

As long as you're replacing a S8 Wedge lamp with one from the same family, it will fit and work just fine. In other words, 3057 works in place of a 4057 or a 3157 in place of a 4157. Personally, I would stay with the correct wattage when ever possible, but if you're bulb fails and you can only get the lower wattage one, it will get the job done.

Go with a Sylvania replacement if you can. When I was an engineer working in the S8 Wedge production area, our lamps had longer lamp life then any of our competitors.

The short of this, a 3057 will be just fine

 

I have a 2007 XLT and few months ago, I replaced the tail light with 3057LL (LL means long life). It works and has no problem.