10W40 too light?

About 1000 since rebuild/teardown,1010 crank, mild racing cam and cam bearings (Crane Cams Anti-Pump Up Hydraulic Lifters, COMP Cam valve springs).
That's all I got from PO who did it. I have some 15W50 I could use but like you say my not be great idea. I don't know what weight oil may have been in there when I got it. thnks for your comments.