In a multi-grade oil the first number is only important for cold starts to reduce engine wear prior to reaching full operating temp. Most people don't realize that even with a 10w or 5w oil you will experience 90% of your engine wear on startup. The oil at those seemingly low viscosities is still FAR too thick to fully lubricate bearings at ambient outside temperatures. Even a 0w oil is too heavy but it beats a 5w or 10w. For an older engine I'd probably look at something like a 5w-30 or 5w-40 type oil. If they offered a 0w-30 or 0w-40 might consider that as well. Even between 30w and 40w oils there is relatively little difference in lubrication qualities and viscosity at 180-200 deg F. Surprisingly, once hot, most quality oils are pretty darn similar. The synthetic blends generally are better flowing at cold temps than dino oils stamped with the same numbers. A 5w-20 synthetic oil might perform more like a 3w-20 when compared to 5w-20 dino oils.

While you might want to raise the hot oil viscosity as the vehicle ages, you don't want to do that with the cold range viscosity. Wouldn't be surprised that in another 5-10 yrs the auto manufacturers are using a 0w-20 or 0w-30 oil in many of their new cars. For the OP's car I'd probably either stick with the 5w-30 or go a bit higher to 5w-40 if that saves on oil usage. Syntec will give you better protection both cold and hot. But I wouldn't go to a 10w-30 as you'll only get more wear, not less. The key has always been getting oil to the bearings as quick as you can for as long as you can as the engine warms up.