A lubrication system for a gear system for a wind turbine is disclosed. The lubrication system comprises a reservoir adapted to contain lubricant, first pump means arranged to supply lubricant from the reservoir to the gear system via a first fluid connection, and vacuum generating means arranged in fluid connection with the reservoir, thereby maintaining a total air pressure in the reservoir which is lower than an ambient pressure during normal operation. The lowered total air pressure in the reservoir draws lubricant from the gear system into the reservoir, thereby increasing the lubricant level in the reservoir and decreasing the lubricant level in the gear system, allowing the lubrication system to be operated in a ‘dry sump mode’. In the case of an emergency or during start-up the vacuum generating means is stopped, thereby increasing the total air pressure in the reservoir. This causes a decrease in the lubricant level in the reservoir and an increase in the lubricant level in the gear system, allowing the lubrication system to be operated in a ‘wet sump mode’.