Using fully synthetic 15W40 diesel oil in your Banshee instead of the usual 10W-30 can cause the issues you're experiencing. Fully synthetic 15W40 diesel oil is significantly thicker, especially when cold, which can affect engine revving and clutch engagement.
Given your Banshee's symptoms, I recommend switching back to the Bel Ray 10W-30 as soon as possible. The proper oil viscosity is crucial for smooth operation, particularly in a high-performance engine like the Banshee's.
As for the oil leak, using a stop-leak additive is a temporary fix at best. These additives can sometimes work, but they are not a substitute for proper sealing. If the cases were improperly sealed, the best long-term solution is to have the cases split and resealed correctly.
In summary, switch back to the 10W-30 oil and consider addressing the sealing issue more permanently rather than relying on additives. Fully synthetic 15W40 diesel oil is great for heavy trucks but not ideal for your Banshee.