Let me clarify a few things. I guess I'm not explaining myself well.
When I said the linetypes load automatically I didn't mean that all the linetypes in the file would be loaded at once. What I meant was if I use the -layer command, or a script, or a lisp routine, or the tool palette to create a layer or object with the custom linetype that it would be found and loaded automatically if the linetype was defined in the ACAD.LIN.
I told the OP to use findfile if he wanted to use my suggestion of adding his custom linetype definitions to the ACAD.LIN file.