I just bought a 95 ranger 4.0 v6 OHV with 95.000 miles, and this is my first oil change, today it's using a 20w50 and I plan to use a 5w30 just like it says in the specification, but it's gonna be a big change of viscosity, and idk if it's a bad idea, what should i do?

Previous owner may be using 20w50 because engine has close to 100k miles

Its a thicker viscosity used in older engines that now have more clearances in bearings and rings so thinner oil passes thru them too quick

5w30 is for new/newer lower mile engines

I would stick with the 20W50 since you don't get really low outside temp in Sao Paulo

Ask the seller why he put 20-50 in it.
On this side of the Panama Canal 15-40 is a very popular "old vehicle" oil selection.
I put 20-50 (a friend's 1985 Subaru spec oil (1.7 liter H4) in my 1976 Chevy Nova (250 c.i.d. 6) and the oil pressure never came up. The engine just clacked.

is very common to people choose 20w50 in brazil just because is cheaper, and truck is used for work so people does this stupid economy, they try to "save money" but when you do stupid things like using a chepar oil, the engine can go down and you gotta spend a big money, people here even use the same oil filter sometimes, is ridiculous. we call this kind of people here as "iron termite"(cupim de ferro( in portuguese)). thank you guys anyways, i am changing for a 10w40 Motorcraft semisynthetic, and a original oil filter motorcraft;
We dont have a lot of options of high mileage oils here.

with this engine I was wondering actually using a thinner oil. Still 5w30, but maybe going synthetic as it seems to run a lil thinner. Common problem on these pushrod 4.0s is not getting good oiling to the top end lifters at either end of the block. Would a synthetic oil reach those corners better? Or is it the opposite, thicker oil that would do better?

I had no idea about oil dry problems with this truck and the 4.0. However, before coming to this post I heard a scratch when starting. Engines got 160 thousand kilometers but is worn in very good meaning my mom somehow broke it in good. No oil consumption, visible main seal leak but in 8000 km I only added a quart, if even a quart, probably closer to 3/4 of a quart. But I heard this scratch on start up. Once or twice in 8000 km. I experimented with a 15w40 and I've logged 1000 km now. Haven't heard the scratch yet. Think my problems fixed.

Important note. The thicker oil seams hard on the battery and the starter. But other than that it's quite normal actually. It doesn't even feel noticeable. It just protects the engine better I think.

I will come back here and post on this once I get some more experience.

I love the term "scratch". I too heard this small noise upon startup.
I use the "clear flooded engine" procedure (ignition ON, depress accelerator all the way down and hold and turn the key to START) for 3 seconds when I start my Ranger and now hear no bad sounds when starting.
On a few occasions I cranked the engine as above until the oil light went out, but was advised that much is not necessary.