Oliveira had been excelling in the wet conditions at Motegi, rising from 16th on the grid to run as high as fourth at one stage after pitting for his wet tyre-shod bike.
The Portuguese rider started to slip backwards, but was still running sixth when he mysteriously pitted at the end of lap 12 of 24, dropping to the tail of the order.
When the race was red-flagged on lap 13 due to worsening conditions, Oliveira was not allowed to retake his position and was finally classified 18th when the race was abandoned following an attempt at a restart.
Post-race, Oliveira admitted that he chose to box because he “couldn’t see anything” in the thick spray.
“It was going well actually,” he said. “I was feeling competitive and with more rain I felt even better. The bike was handling well. But three laps before I retired I started to struggle to see.
“I couldn’t see anything, I couldn’t even see where I was going to put my wheels. There was so much water, I was telling myself not to quit and stay calm, but I came in a lap too soon.
“This never happened to me before, but I couldn’t see a thing - braking, in the corners, even trying to stay out of the spray. It was really frustrating.”
Honda rider Joan Mir experienced similar visibility problems due to an issue with his helmet, dropping from seventh to an eventual 12th.
“I never had this in the past but somehow today was the day,” said Mir. “I couldn’t see. I was missing the reference points on the braking.
“When I was fighting with Jack [Miller], I could see a bit, but then the rain increased and it was enough to make it fully wet inside [the helmet] and it was impossible to see.
“I’m very disappointed. We have to understand why this happened and that’s it. I am sure that for [helmet supplier] ATV, they always make a huge effort to avoid this, so together we will find out why this happened.”
Fernandez calls long-lap penalty a "joke"
Oliveira’s RNF team-mate Raul Fernandez meanwhile was left incensed by his long-lap penalty after ending up ninth in the shortened race.
Fernandez was awarded the penalty because he didn’t pass the correct side of the number marker that divides each pit box in the pitlane when he came in to change to his wet tyre-shod bike along with the majority of the field at the end of lap one.
“I am very angry,” said the Spaniard. “We are in the best championship and the rules that we have are a joke, honestly. We need to improve the f***ing penalties.
“You kill another rider and you get a warning, but if you close [tighten] the line and cross the pit box before yours, you get a long-lap penalty. For me it’s a stupid thing.
“We did a great job, we were fighting for P6, but we need to improve the penalties. Sometimes it looks like we are children.
“Maybe my opinion doesn’t change anything, but I have to say it.”
