Chevy has been using o-rings for decades... An some will say Teflon doesn't allow enough oil to the guide... Simple is what simple does... They work, I still build my Blower engines with o-rings..
I like Teflon, more spring and collar clearance, last longer and they gave me 1200-1300 miles to a qt. of oil. Of course, the guides must be cut to fit. Had nothing but trouble with umbrellas. Only got 500-600 miles to a qt.
I started using the Teflon type seals in 1966 and have never had a valve seize in the guide as some say will happen. The stock umbrella and "O" ring will work too but they do get brittle and break from heat and age. Be very careful putting the "O" ring in place it is easy to cut it when you release the compressed spring.