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Don't make the mistake of thinking that at 180,000 km you can use just any oil or, worse yet, a thicker one thinking it'll "seal better." At this mileage your engine already has normal wear within tolerances, so what you need is to strictly follow the manufacturer's specifications - that SAE viscosity and ACEA classification you'll find in the manual. Changing oil without following what the owner's manual says risks only creating problems.
That said, it depends on what the manufacturer recommends. If the manual says 5W30, stick with that; if it says 5W40, don't improvise. The important thing is choosing quality oil - not the cheapest one you find - and respecting the service intervals. With the mileage you've got, a synthetic or semi-synthetic oil is more suitable than pure mineral, because it holds up better over time and protects engines that already have years on them more effectively.
Also check the general conditions: if the engine is burning oil, if it smokes a bit on cold starts, or if you keep getting the warning light on the dashboard, it might mean it needs more attention. In that case a visit to a trusted mechanic before choosing the oil could save you headaches.
At 180,000 km, the manufacturer's manual is still your bible, because that's where you'll find the exact viscosity your engine was designed for. If the service booklet says 5W-30, stick with it - don't make up alternatives. I've seen plenty of people convince themselves that with wear you need thicker oil, like 10W-40, and the result is the engine runs worse, burns more fuel, and the internal components (oil pump, filters, galleries) suffer more because fluidity drops.
What really changes at high mileage is how often you change the oil. Oil consumption goes up, protective capacity degrades sooner, so instead of switching to "stronger" oil you should check the level more frequently and maybe move up the oil change by a few thousand km compared to the standard intervals. A quality synthetic or semi-synthetic oil (one that meets the manufacturer's standards, say an ACEA C3 or higher if your engine allows it) protects you better over time and maintains its properties better between changes - exactly what you need at this mileage.
The oil pump, bearings, piston rings: at 180,000 km they've already worked hard, so the worst thing you can do is run an oil through them that doesn't flow the way it should or doesn't have the right detergents to keep the engine clean.
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