I have a curious question, that maybe an expert vaper might be able to answer;
I normally buy Double Drip brand liquids (Pre-Lockdown). A coil only lasts about 10 days and it burns out and when I change it out the cotton is black.
During the lockdown I purchased a Litre of VG and PG and Flavour Concentrate and made my own, the coil lasts about 6 weeks before i have to change it and even then the cotton is only yellow (would of lasted even longer). This is not just on one coil i have since gone through several of the past months.
All i can think of is that maybe Brand Liquids contain other additives that are not listed on the bottles.