The oil needs to be warm, but, even better, hot, especially when riding at speed. When cold oil is pumped into an engine, cavitation (bubbles of vacuum within the oil) is likely to occur. This, in essence, means that the engine does not receive enough oil for it to run optimally at speed. Warmer, and so thinner, oil ensures that the engine not only receives enough oil, but that all moving parts within it can work optimally. Ideally, use a 5W-40 or 10W-40 oil and ensure the engine warms up properly for a few km before speeding up.