A mixture of bases is commonly known as a wobble base. Please find the universal nomenclature (IUPC code) in the following table. Just include the respective character in the oligo sequence.
A wobble is generated by equal amounts of different phosphoramidites in one coupling reaction during synthesis. Not all nucleosides are equally incorporated during synthesis; it is not uncommon to observe a slight bias at the wobble position in the final product.
In addition to standard wobbles (i.e. Y (CT) 50:50) you can also order wobbles in a defined ratio (i.e. CT 30:70) by choosing "wobble defined ratio" in the drop down menu within the "internal modifications (X)" section in your oligo order form. Please define your requested ratio in the comment field (e.g.: X=Y= 30:70).