Switching from 20W-50 to 15W-40 might result in slightly thinner oil at high temperatures, potentially offering less protection under extreme heat and pressure. However, if you’re changing the oil after every run, the difference may be minimal for a single pass down the track. The advantage of 15W-40 could be better cold start performance, but 20W-50 is generally more suited for high-stress conditions. Just like using the correct "i-shift transmission oil" is crucial for transmission performance, ensuring you have the right engine oil for your specific needs is important for optimal protection.