An experiment on System`Private`InternalSeries in Mathematica v5.1+.

test-InternalSeries.m

(*
 * Tested in v5.1, v5.2, v6.0.3, v7.0.1, v8.0.4 and v9.0.1.
 * It is said that Wolfram changed the way how Series works between v5.0 and
 * v5.1, but I don't have any working binaries of <= v5.0 for now.
 *)

ExplicitFunc[x_] := 1/x^3 Log[1-x];

Series[ExplicitFunc[x], {x, 0, 3}] // Print;
Series[Log[1-x] ExplicitFunc[x], {x, 0, 3}] // Print;

(* Define the power series via Series. *)

Func /: Series[Func[x_], {x_, 0, n_Integer}] :=
  Series[ExplicitFunc[x], {x, 0, n}];

Series[Func[x], {x, 0, 3}] // Print;
Series[Log[1-x] Func[x], {x, 0, 3}] // Print;  (* Func[x] is Taylor-expanded. *)

Clear[Func];

(* Define the power series via System`Private`InternalSeries. *)

Func /: System`Private`InternalSeries[Func[x_], {x_, 0, n_Integer}] :=
  System`Private`InternalSeries[ExplicitFunc[x], {x, 0, n}];

Series[Func[x], {x, 0, 3}] // Print;
Series[Log[1-x] Func[x], {x, 0, 3}] // Print;  (* Correctly expanded. *)

Clear[Func];

(* vim: set ft=mma et ts=8 sts=2 sw=2: *)