Will this jack fit under my car with lowered suspension?

The minimum lift height is 5.3", which works for most lowered setups. If your car is slammed lower than that, you'll need to measure your pinch points first — don't force it under if there's no clearance.

Do I need to add hydraulic fluid or is it pre-filled?

Pre-filled and ready to use out of the box. No fluid top-up needed before your first lift.

Can I use this on uneven ground or concrete?

Concrete is fine. Uneven ground is a safety issue — always use this on flat, stable pavement. The base isn't designed for rocking or settling, so level surface is non-negotiable.