Let see the why would the Germans attack Romania (or any other country) for territory or for resources,because is a threat (here i include revenge),out of the three only one can be valid for Romania,resources so Romania will have to stop supplying oil.
So why and when would Romania stop supplying oil,if she can not,or if UK and France are offering support,with troops on the ground and planes in the air,in which case the Germany has a big chance of being alone since i don't see the Stalin or Horty joining in with France sill alive and UK on the continent.
And then we have the when,and since there is no direct border between Romania and Nazi Germany the attack will probably come in case of a war between Czechoslovakia, or if Romania becomes involved in the Hungary-Slovakia war,or after the invasion of Poland,if it is the first chance are Romania is not alone,in the second one chance are Germany will let Hungary go,no to mention that in both case Romania will have to have a very bad relation with Germany,so we are left with the third option after Poland.
So we have a Romania that has a bad relation with Germany,and is supported by UK and France.
As for the German objective in the invasion will be to secure the OIl infrastructure as intact as is possible,and for that Nazi Germany will have to be able to secure air superiority and drop in paratroopers to secure the oil infrastructure,Germany will need to do this no mater how well equipped and trained is Romania.
So for an war to happen between Romania and Nazi Germany there must be a very bad relation between the two and a Romania that is supported by France and Germany,and by support a mean very thing from quipped to trained to boots on the ground.
And of then there is the question of from where ? With Hungary neutral the attack can come only from the north,if the Soviets leave the Germans a corridor of if Hungary dose not take all of Ruthenian,let say that both are leaving Germany with a route to attack,the bad think for the Germans is the they will have to cross a few mountains and hills before the will reach the oil,which will have to be secure in mean time.