Oil Viscosity question

[Flathead]

My engine is tired. How thick can I go on the oil (warm weather operation) and still have it flow down the tubes to lube the main bearings etc. The engine is not full of sludge, I have had the oil pan and side cover off already. I would like to get a little more use out of it while I finish rebuilding another to replace it. Interested in other people's experiences. Thanks.