Was never a 5w though. Id use thicker mate.
You obviously dont understand oil viscosity, because for all intents and purposes for the majority of engine runtime 5w50 is the same viscosity as 10w50. I will attempt to explain with some text plagiarised from the internet, but dumbed down a bit just for you
Viscosity (a fluid's resistance to flow) is rated at 0° F (represented by the number preceding the "W" for Winter) and at 212° F (represented by the second number in the viscosity designation). So 10W-30 oil has less viscosity when cold and hot than 20W-50. Engine oil thins as it heats and thickens as it cools so with the right additives to help it resist thinning too much, an oil can be rated for one viscosity when cold, another when hot. The more resistant it is to thinning, the higher the second number (10W-40 versus 10W-30, for example). Within reason, thicker oil generally seals better and maintains a better film of lubrication between moving parts.
At the low-temperature end, oil has to be resistant to thickening so that it flows more easily to all the moving parts in your engine. A 5W oil is typically what's recommended for winter use. Once the engine is running, the oil heats up. The second number in the viscosity rating--the "40" in 10W-40, for example--tells you that the oil will stay thicker at high temperatures than one with a lower second number--the "30" in 10W-30, for example.
So, I suggest a 5w50 because the 5w means its fairly thin when cold, comparatively, and flows out to the nooks and crannies much easier which is good. The 50 then means that as it heats up it still retains a bit of 'thickness' instead of going like water and we need that for these old, lower tolerance engines that have been running for 20 years. In my 172 which is a more modern engine I use 5w30 which is the same cold viscosity but thinner hot. Remember that the two viscosity numbers are
unrelated to each other. They give a cold viscosity and a hot viscosity.
We can go on if you like and talk about detergents, dispersants, antiwear agents, friction modifiers, pour-point depressants, antioxidants, foam inhibitors and corrosion inhibitors
Also bear in mind the way oil technology has improved over the last three decades, and how much better our synthetic oils are now.