Previous engine builds (lotsa Pontiacs, coupla Olds's, few 289's & 302's), used teflon or teflon insert valve stem
oil seals. This was SOP. The rubber seals with the gasket set were trashed. The rubber ones harden after awhile, then the engine starts drawing
oil down the valve stem. Not a good idea for a warm street motor.
TFS recomends using the stock type neoprene rubber valve stem seals stating the valves need more
oil. Machinist doing heads dutifully removed the teflon seals and put the rubber ones on. Has iron, not bronze guides.
Some of you have more miles on your TFS's than I do. Teflon, or rubber valve stem seals?