Mine does the same regardless of oil brand. The clutch springs as standard are adequate, but as they age the free length reduces until they don't quite supply the pressure needed when the oil's cool and has higher viscosity.
The other factor is wear and contamination of the clutch plates. If oil with friction modifiers has been used you might as well change the friction plates and springs, otherwise just change the springs.
Don't be tempted to buy aftermarket springs: they're invariably heavy duty, turn the nice light clutch action into absolute misery in town and overstress the pressure plate and actuator.
If it's the first time the clutch cover has come off you'll need a new gasket, but if you grease one side of the new one (not with moly grease!), you'll be able to take the cover off in future without wrecking the gasket.