Welcome aboard csmadore from southern NH! Where in Maine are you? I lived in E. Eddington as a young kid in the early 60s while dad was the service manager at the Bangor Sears.
That Penzoil is most likely the cause of your clutch slipping. Penzoil has friction modifiers and that causes the clutch to slip in wet clutch bikes. It is fine for car motors as clutches on them are dry clutch systems but not good for wet clutch bikes.
I learned the hard way too using it in my 83 goldwing when I got back to riding in 2009 from a 25 year away from bikes. I bought the bike that had slept for 13 years or more and brought it back to life. The clutch slipped on my 1st ride.
I also learned from that asking on the goldwing forum I was on, that Seafoam and Marvel Mystery Oil both can be used to flush the crankcase of the grunge of old oil left in the motor of the bikes for years, and if done properly it will clean the clutch plates of the friction modifiers. So I did it according to the forum advice of long time members there to put in Rotella 15w40, put in 2 qts of oil, then put in a full can of Seafoam, 16 oz, then fill the oil level to full mark with the Rotella. Go out for 1 ride, of 100-200 miles, keeping the rpms below 2,500 so you don't harm the motor works, shifting lots up and down to clean the clutch plates and circulate the oil in the clutch housing as there isn't a lot of oil flow through the clutch area. Again, don't hammer the motor. The 100-200 miles will fully heat the motor and any grunge in it so that when you get back home, drain the oil while it's still hot so all the grunge will flow out with the oil. Change the oil and filter. Put in fresh Rotella.
Then you can ride it for another 1500 miles as normal and then change the oil and filter again. Then do normal oil changes at 3,000 miles after that. I put 26,500 miles on that wing in a year and a half with no problems, and didn't have to change the clutch in it.
I've used the Rotella in all my bikes, wings and CB750s, and also have flushed each of the old engines with Seafoam to get the old oil grunge out. I haven't bought a new bike since 1976.
For my 750 I run Honda 20w50 oil in the summer months and Rotella 15w40 in cooler months for easier starting. I usually put the bikes away mid November for the winter nap with a fresh oil change after the last hour ride to warm the bike 1st.