Thanks everyone for your answers and comments. I went to the dealership Friday after work to sit on a stock Road Star seat. Yes it's softer than the Mustang, but I noticed immediately what Erbman02 said: absolutely no back support. Then on the next day, I rode about 120km and the back support the Mustang gives is very noticeable. So I made my choice to go on with the Mustang and save money for future mods...or repairs
Yeah, 2 weeks ago a rode to work but got caught in very heavy traffic when at some places it took about 15 minutes to go 1 or 2 kilometers. So there was alot of 15-20 feet stop-and-go. So no need to say that the oil went very hot. Back at home I noticed oil drops and spill on the bottom right side below left tappet block, decompression solenoid was all oily and everything under with oil drops going under the frame towards the back. I cleaned it all and coming back from yesterday's ride, I took a look in the same area and same story. I'm afraid a seal may be leaking due to the overheat, or something else, I really have no idea. This is my first year with my Road Star (and first year with a cruiser) so I didn't had much occasion to learn the basics of that bike, the components, etc... If anyone came accross that problem, what was the cause? What did you replace? Well, maybe I should start a new thread for that topic and add pictures... I'll see if replies come.
But thanks for your comments on the seats... and the horse riding