Hi,

I just bought Pontial Grand Prix 1994. I have found in maintenance record that previous owners were using 15W40 oil from 2008 (125K miles) to 2018 (155K miles). After 2020 car was stored in garege.

Recommended oil is 5W30. My question is if its good idea to clean engine with cheap recommended 5W30 and engine flush additive and idle for like 15mins and after that do change for better 5W30.

I wouldn't worry about all that. Just put good 5W-30 in it.

The residue of 15W-40 left over in there won't hurt anything.