1997 Plymouth Voyager Engine Stalls while driving (under lo

I have a 1997 Plymouth Voyager 3.0 Litre Van (base model). I got the van a little over a year ago and the van has always run great. It does at startup after it sits for a night put out about a second or two of dark smoke - and I have to top up the oil every three to four weeks. So I know I have a internal leak.
The problem is after I did an oil change during the winter - A oil tech said I should be putting in 10w30 in instead of 5w30. So we went with that. Two days later the van stalls while driving and I can only keep it going with the foot on the gas keeping it at a higher rpm. I was at a loss. The van had no trouble codes, and would just stall - the radio and all the lights would be on but the engine would stall. I noticed after a couple of stalls that the oil light would come on for a second then turn off.
So I complained to the oil tech - He drained the 10w30 and put 5w30 back in with a new filter.

This seems to 'fix' it. But why does it do this? Can't this engine run on 10w30? Also whenever a thicker oil additive or 'cleaner' is put in - a couple days later the stalling starts again. I was hoping the thicker oil would slow the leaking. I think it's the oil but maybe there is another issue?

Do you have a hypothesis on this? Or maybe an idea for a fix?

thank you

-Ken