This sounds more like a gearbox that isn’t selecting fifth gear properly, rather than one where it’s leaping out of fifth. For it to pop out of gear when cruising along suggests it’s not getting into gear fully in the first place. You might be able to adjust the shifter’s position and alignment and therefore get fifth gear to engage more fully.

There’s also a chance that this refusal to select fifth gear is the result of bent or worn selector forks within the transmission itself, and that’s a gearbox-out job unfortunately. So the first step is to get underneath with a torch while somebody inside attempts to select fifth gear. If the selector linkage runs out of range of motion, you might just have an adjustment problem which can be pretty easily sorted.

You should be running a 5W-40 oil.

The Ranger, and its cousin from Mazda, is a tough, durable and reliable truck. Neither have any major problems that should concern you. Buy on condition, look for one that has been used privately rather than for work, and make sure it’s been well serviced.