Syntax for if/else condition in SCSS mixins

I created a SASS @mixin wich contains @if conditions to assign styling to elements based on their z-index property to create some sort of elevation.

However what ever I am trying It will not work out.

I'm pretty sure I am doing something just slightly wrong that affects everything else.

I'd appreciate your feedback. Thanks in advance!

$background: #121212;
$surface: #1f1f1f;
$surface-shade_1: #282828;
$surface-shade_2: #303030;
%surface {
  background-color: $surface;
}

%surface-shade_1 {
  background-color: $surface-shade_1;
}

%surface-shade_2 {
  background-color: $surface-shade_2;
}

@mixin elevation($elevationValue) {
  @if $elevationValue>0 {
    @extend %surface;
  }
  @else if $elevationValue>4 or $elevationValue=4 {
    @extend %surface-shade_1;
  }
  @else if $elevationValue>8 or $elevationValue=8 {
    @extend %surface-shade_2;
  }
  z-index: $elevationValue * 50;
}

nav {
  @mixin elevation(4);
}