I meant mixing it with normal oats and using the same 28g in a day or is it better just taking the full syn hit
Unfortunately you can only use the Healthy Extra's as they are listed by SW. There is no mixing or substituting out of that list. The healthy extras are carefully chosen by Slimming World by a number of factors.
It is a case of if you want to be 100% on plan or not. If we were able to chose our own Healthy Extras, then there wouldnt be much point in the list.