I have a hard time believing choice of oil would have anything to do with the gasket.
I put more faith in the words of
@sonny ,
@Speedy ,
@Rocket Scientist ,
@warp9.9 -- I liken it to making southern biscuits.
A recipe might say, "Mix 2 cups of flour with 8 tablespoons of butter, and add a cup of buttermilk. Make 2" rounds, and bake at 425ºF"
There are 10,000 or more ways to do that, and NOT have biscuits. There are a mere handful that yield delicious biscuits of the correct consistency.
The devil is in the details here.
Is the gasket flat ?
Is the head flat ?
Is the head scrupulously clean ?
Have the bolts been lubricated ?
Can the cover be placed on the head without dislodging the gasket ?
Do you have the patience to torque the bolts in sequence ?
Do you have the clearance to actually get a torque wrench on each bolt ?