Smoke/No Smoke from '94 XJ6

Can someone tell me if this makes sense? I have a '94 XJ6 that just turned 70k miles. A couple of oil changes ago, the shop put in 10w-30 instead of 20w-40.
I started getting large puffs of smoke each time the car was started. From this forum, folks told me it came from oil leaking down the valve seals when the engine was shut off and not to worry about it. Last two oil changes I made sure the shop went to 40w, and now the puffs of smoke are gone. Could this be attributed to the viscosity change? If it was seeping past valve seals, I would think 40w would seep as much as 30w. Help please??? Paul
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The smoke occurs on cold start before the valve stem has expanded from the combustion heat. When cold, thinner oil will be sucked into the intake via valve stem seals than thicker oil. Stay with SAE 20 - 40 or 20 - 50.
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Thanks for your feedback. Yes, I intend to stay with 20-40 from now on. It is surprising to me that the difference in viscosity between 30w and 40w would result in the big puffs of blue smoke upon starting. Not just starting cold, but re-starting a warm engine after letting it sit for only 15 mins or so.

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