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 modifications drop down. Please define your requested ratio in the production comment during checkout.