Saw this post tonight…..

Mixed synthetic oil with conventional oil! Need help! – BMW 3 …
Mixed synthetic oil with conventional oil! Need help! Turbo / FI Engine and 335i -335d Tuning / Drivetrain / Exhaust Modifications.
www.e90post.com/forums/showthread.php?t=395058

Once again, there is NO risk to mixing synthetic and non-synthetic oil. I’ve done it in emergencies, and it won’t hurt a thing. Also, there are now several “para-synthetic” or partially synthetic oils on the market. This is basically synthetic mixed with petroleum, so how could it be an issue?

Years ago, before Amsoil came out with Amsoil engine flush, we used to put 1 quart of synthetic in with an oil change to clean out sludge prior to changing to Amsoil. It worked great. No problems.