After every event which could be less than 300 miles! I use 5w40 Fully synth, which I buy when it is on offer from ECP, works out about £15 for 5Ltrs. I also only use either Nissan OEM or Blue Print filters.
On a road car, I would use 10w40 probably Vauxhall trade stuff as you can buy it in 20ltr drums for less than £40.
MGZR gets the Vauxhall oil but Rover filter, and is changed about every 6 months, as it is a standard engine.