A determining factor is the age and size of the engine block. I have a 1968 428 Cobra Jet in mine and I tried running 10-40 and it did not work so well, and was considering going to 10-30, but listened to a few people and went with the Valvoline 20-50 racing
oil and the engine loves it.
Oil pressure is only slightly higher than with the 10-40, but
oil consumption is down and the engine runs smooth. I have solid lifters.
With a newer block or a small block, a lighter weight oil will probably be fine and I would follow Bob's advice. Unless you are running an older fe, or are racing hard, you probably do not need 20-50. Each engine is different and sometimes you have to experiment with different oils and viscosities and see what works best for your engine.