I don't know how much driving you do in Panama but a 1995 car with 75k km on it sounds too good to be true. In Malaysia we cover that kind of mileage in a single year.
My car is a 1994 with 170k km on the clock and I use Castrol's GTX which has a 15W-40 SAE rating and it works fine in hot tropical (and bloody congested) Kuala Lumpur.
I've used Shell Helix (all versions from the mineral to the synthetic) before and I'm not awfully impressed with it. I'm sure Michael Schumacher loves it but I'll stick to my humble Castrol GTX for now.