1997 5.7 mainbearing bad?
 

I have a 1997 5.7 with 102000 miles on it. I bought it with 92000 and engine appeared fine. I am having to have the gear box replaced (probably due to previous owner handling) and the mechanic noticed a knocking in the engine. I have been noticing it especially when I first crank it up. He said it sound like the main bearing. Told me to put some 20w50 in it and if it appears to help then it is most likely the bearing.

Is this really likely? My last chevy (I bought new) had 230K miles and no issues!

Thanks for any help
Greg