Yes, I need to take it up to 75-80 kmph on the highway, with headwind it won't go past 70 kmph though, and let's not talk about uphill

The km should be correct, this van is an old ambulance from the civil defense. It's been stored in the mountains for a long time because they have been replaced by newer vans, and the previous owner had just let it sit outside for 4 years. So could lack of use cause extra blowby?

Maybe I should switch to synthetic oil and drive it hard for a while? I can get the Mobil Delvac 1 5W-40.