Crank rattling and oil rating

[LinkMJB]

Hi, I just changed my friends oil today on his 1995 Nissan 240sx. The guys at Autozone told me to use 20w50 for a car with it's amount of miles (250,000) so I went with Valvoline 20w50 (the special blend for high mileage vehicles) and replaced his oil. They said to replace it with 4 quarts because it's a 4-cyl. So, I remove his oil and it's thick as sludge and only amounts to 1 quart. I remove his old oil filter, replace it with a new one and fill up his crankcase with the new oil. His car runs better than before we go pick up some pizza, then come back and park. We then go back out a couple hours later and now it's making rattling sounds (directly under the oil cap, I assume it's a rod from reading other posts due to low oil pressure). Now, how can I fix this problem before it turns into something really bad? I won't let him drive it because I don't want the rod to get screwed up. Is the weight of the oil improper? Did I not use enough quarts of oil? Also, why did it take a couple hours to tell that it has problems... why not right after the oil change? If anyone could help, I would really appreciate it. What weight oil should I use, and how many quarts? Thanks a lot, and hope to hear from you all soon.

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[benemorius]

So how much did you actually put in it, and what does the dipstick show now?

It sounds like it needs to be de-sludged pretty badly. Search the forum for info on sludge removal.

If the oil was as bad as you say it was, the noise may have shown up simply because the new oil cleaned up some sludge that had been dampening the noise all along.

Four quarts is good for that engine, but in the future, come to the forums and do some research rather than taking the word of the dude at autozone. I'm not saying don't listen to him; I'm just saying compare what he has to say with what others here on the forums have to say before accepting it. His suggestion to use high viscosity oil is a good idea, in general, to slow oil leakage/consumption on high-mileage engines, but I would at least give it a chance on a more normal oil and see how it likes it first.