There is a lot more to being a commercial brewer than just making beer.
First of all you need an ATO license and also council approval. Both require a great deal of faffing about.
Are you measuring PH before adding lactic acid? with a reasonable proportion of dark grains you should not need to be lowering PH it would be more usual to be looking at raising PH. I am not necessarily saying that is the cause of the oil slick