Agreed ....colder climates would better with 5w-30 or 5w-20
In warmer climates that do not drop below say 50-60deg ,ever...maybe 10w-40 oil would be better choice.
you certainly can try the synthitic 5w-20 if you want....but thinner or lighter weight may not the direction to go if your air temps tend to run a bit on higher side on a consistant level.
I suggest to try regular dino oil in the weight your considering 5w-20 1st. One, it is cheaper and if it doesn't produce the desired result, (less valve clatter in your case) then you're not out the high priced oil and run it a few hundred er so miles and try a different weight. wouldn't go any further than 15w-40 though, any more than that could be too thick and create more wear (IMO) exspecially on start up, when most wear happens anyway. if things improve to your likeing then upgrade to the synthitic.
Another reason is that you're engine ,in all likelyhood, has slude /gunk build up that is restricting flow to the upper end, hence the clatter...with a few, realatively speaking, rapid changes on the oil and filter testing different oil weights....may...break down this restriction and carry it out into the oil and filter. and possibly improve things for you.
the reality is that, -sorry to say- that your engine is need of valve job or rings or some kind of solid repair and changing oil wieghts until you get things to quite down is band-aid solution. It might help in a short term but in the long side of things is still going end up doing the repair. sorry to throw a wet blanket just I have not had a great deal of success with this approach. If it works for you, SUPER!