Where should heat comes from when baking brownies?
Fan ovens are generally regarded as 10% hotter than regular ovens, so I would suggest fan only, dropping the oven thermostat setting down by that amount than your recipe dictates. Fan mode will give you a more even, consistent temperature throughout the whole oven, whereas the different elements (and non-fan mode) will bring different results.

Using the top and bottom elements will concentrate the heat at the top and bottom of the oven respectively, and will also affect the texture of the top and bottom of whatever you are cooking. Non-fan may lead to hot-spots or the heat concentrating around the elements.

I find turning all the elements on at first to quickly preheat the oven, then switching to fan gives good results, but I have a very large 90cm single oven.

https://hummingbirdbakery.com/blogs/hummingbird-bakery-blog/oven-temperatures-and-baking-explained