Rod Knock when cold - Ever had this happen?

[Yamariv]

My L6 250 seems to have developed a rod knock on startup when cold, once warmed up though (2-3mins) it goes away and the engine purrs like a kitten and sounds and drives great. I was running 5w30 oil in the winter and switched to a 10W30 hoping this would help but it hasn't. I just took it on a 350 mile road trip to pick up my future 350 and it didn't miss a beat.

Anyone ran into this before, it's definitely a knock. Can knocks go away like that after warm up? Should I jump to a much thicker oil like a 20W50. I was planning on driving my L6 250 for a year or so while I rebuild my 350 so I hope to extend it's life until its ready.

Cheers