Do anyone why have any idea to stop the engine oil seepage from the
engine oil cap? Is there anything to do with switching from regular oil
to synthetic? Thanks for any suggestion.

I may be due to excessive crankcase pressure. Do you know about the "flametrap" or when it was last serviced? Check the 700/900 FAQ Feature for details (pull-down menue at the top od the page).
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Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Sold 5/03)
'83 Turbo 245
'76 244 (lasted only 255,000 miles)
73 142 (98K)
'71 144 (track modified--crusher bound)
New 144 from '67 to '78
Used '62 122 from '63 to '67

Replace the gasket under the cap. If that doesn’t totally solve the problem you can bend the two tabs that hold the cap in place, toward the inside to provide a little more clamping force when the cap in place. Don’t bend it too much, 1/32-1/16 should be enough. FCPGroton.com has “oil filler cap seals” for .65 cents.
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Tom F Three 940s. Lein holder on two more (the kid's). Rust in peace, '78 245, probably Campbell's soup cans by now.