This month, an update from Bernina for the owners of 7 series and 5 series machines. Bernina now recommends that you no longer place oil in the reservoir as described in your user manual as this has caused oily threads and discoloration of fabric. 

Don't oil Bernina reservoir

Instead, oil your machines' shuttle race as shown in the machine's Help Screen.

oil here 

We had many cases reported, where extensive reservoir filling caused oily threads and discoloration of the fabric. A number of our customers locally experienced this when making free standing lace on their 7 series machines.

You should place a drop of oil on the shuttle race every 2 bobbins or before sewing if your machine hasn't been used for an extended period of time.