knocking lifters

Sunday on my way home I was running low on gas, went bought some and not about 5 min later my truck was making this knocking sound, blamed it on the gas first, only bought $5.00, filled up my tank with better gas put
some STP in it, it wasn't the gas, the only knock real bad in low RPMs, it is a 1990 gmc truck, any ideas
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Sounds like your oil pressure might be low. You should have put STP (oil treatment) in your oil instead. Or maybe at least you should check the oil level to see if you have any. PS. Thanks for stating the year of your truck. That way we can estimate the mileage as anywhere between 2 and 2,000,000 miles.
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