I have been using Pennzoil Platinum 5W30 Full Synthetic and the OEM blue filter. Changed to this from whatever conventional oil my (former) dealership put in. Smoother starts in the winter and (I think) a smoother idle is all that I noticed. I'll try 5W40 next.
Speaking of 20W50 synthetics, I use 20W50 Mobil 1 in my Harley's V-twin. Maybe tests prove otherwise, but I'm not confident about putting that into my LGT. Just me...