Not ideal but not horrible. The 5W means they have the same cold viscosity but the 30 vs 40 means they have different hot viscosities. As the first source puts it, "Think of it like this: a 10W30 oil is a 10-weight oil that will not thin more than a 30-weight oil when it gets hot."

Where the difference comes into play is in oil flow. 40W gives higher pressure but less flow. Your 2000 Prelude has bearing clearances spec'd at .001-.002 inches, just right for 30W. 40W engines are typically .002-.003 inches clearance. Contrary to popular belief, bearings simply do not wear measurably. If one starts wearing it deteriorates rapidly, leading to our old friend rod knock. I tore down a Volvo engine with 190K miles on it and the bearings had their original coating of indium (most bearings use babbit metal, but not Volvo in that vintage), which meant it was not worn even .001 inch. So 5W-40 will reduce flow, about like you would expect from lower pressure.

A higher viscosity oil will break down LESS then a lower viscosity oil. Cold weather starting is not an issue. The first number indicates cold temperature viscosity. The second number indicates warm weather viscosity. The only time this can be an issue is if you have a turbo charged motor. The heavier oil might slow the turbo spin down a little.

Using a higher viscosity oil in an older engine helps oil pressure and makes for a better film between piston rings and cylinder walls.

On the flip side, thicker oil flows slower so the oil and engine run hotter which breaks down the oil faster, turning it into sludge. That also wears out your engine faster.

Go back to 5W-30 but switch to synthetic High Mileage oil. It has additives to get you better film strength and added blowby gasses. Read this article on oil for high mileage engines

http://ricksfreeautorepairadvice.com/wha...

It depends on the area you live in. The w40 or w30 pertains to winter weight....meaning how thick the oil gets in cold climates. If you live in an area where it gets really cold then the 5w40 can become thick and may be harder for your engine to handle. However it is not much of a difference in viscosity and you should be fine. If you are so concerned go to a lube center, have them flush you engine, and then ask for 5w30.

I have a 2000 Honda Prelude with about 190,000 miles. The manufacturer calls for 5w30, but recently I have started to use 5w40. Can that potentially cause harm? Is it ok that I am using 5w40? Thank you.