Draining via the sump plug ensures that the sump will be empty.
Why not just measure the correct amount for a fill rather putting back what you removed? Following is service bulletin SSM49357 from Topix as reported by a Disco3.co.uk member:
Models: Discovery 4/Range Rover Sport
Title: 3.0 TDV6 Engine Oil
Last modified: 10-May-2011 00:00:00
Symptom; 401000 Oil System Concerns
Issue: Clarification on engine oil specification applicable to 3.0 TDV6 derivatives.
Cause: Incorrect engine oil specification being used in some Markets.
The engine oil used in 3.0 TDV6 must be to specification WSS-M2C934B (ACEA A5/B5/C1). Land Rover recommends Castrol Professional Powerflow C1 5W-30 especially for vehicles fitted with DPF (Diesel Particulate Filter) as this engine oil is specially formulated to minimize ash build up.
For non DPF vehicles ONLY, engine oil to specification WSS-MSC913B (ACEA A1/B1) may be used. Land Rover recommends Castrol SLX Professional C1 5W-30. It is permitted to use an alternative to Castrol as long as the specification stated above is met. The grade of oil used must be 5W-30, there can be no deviation from this viscosity. Failure to use a 5W-30 viscosity engine oil meeting the above stated specification will have a significant effect on the bearing shells which may cause irreparable damage to the engine.
NOTE - It is imperative that when the engine oil is drained from the sump during routine service work and replaced, the correct amount of engine oil is added to the engine as follows - 3.0 TDV6 Engine oil service fill with oil filter change 5.9 litres – Maximum. Therefore it is essential the correct amount of engine oil is measured prior to filling.
Failure to follow the above instruction will compromise the engine breather system, potentially resulting in oil pull-over and subsequent engine damage
MY12 RRV 4.4 TDV8 AB, LT275/55R20 BFG KO2, LLAMS
Produce LLAMS for D3/D4, L320 & L494 RRS, RR, Jeep GC & Dodge Ram