I flew back from Sao Paulo to London this week on BA246 with a colleague who would be my lounge guest. There is no dedicated BA lounge at GRU airport but the BA website indicates three choices of lounge for oneworld passengers; LATAM VIP Lounge, Executive Lounge GRU Airport and American Airlines Admirals Club.

At check-in we were asked if we knew the lounge location and were advised the lounges are all together on the third floor after security and immigration and that BA passengers preferred the LATAM lounge although we were free to try all of them. We headed to the LATAM lounge which is large with plenty of seating, hot and cold snacks, the usual coffee, etc, wine and only Heineken beer. Unfortunately the lounge is not very comfortable and feels huge plus was quite busy. We then moved to the adjacent American Airlines Admirals Club and what a difference. This looks brand new, again is large with lots of seating, all with power points and USB. Again the usual hot and cold food and a wide selection of beverages. Both lounges had good toilets and showers and WiFi.
After an hour or so my colleague mentioned the lack of any other passengers - we had the whole place to ourselves and only two other passengers eventually arrived before we departed for the gate.

The BA flight was pretty busy, Club World completely full (not sure about First) and also definitely other Gold and Silver ECs about. So we have to assume they all use the LATAM lounge as recommended by the check-in staff even though we would recommend the AA lounge. Maybe LATAM benefit by charging BA for each passenger that enters their lounge and this is why the local staff recommend this option?