polaris states that you can upgrade any of their oil at any time. all of their oils will mix together. When I changed from polaris gold to amsoil I ran tank down to low line and then filled up with amsoil. That was before ves oil. Some oils can have chemical reactions and get thick but their oils will not.

 

I think if you are using TCW 3 oils, adding one to another you'll be fine. The TCW 2's and before that sometimes had bad reactions with each other. If its adding one Polaris oil to another, you'll be fine.

 

I dumped Series 2000 Amsoil in my 03 from new and it is just fine. Smells better too.

[edit]My point was mixing is fine