After topping an opening day curtailed by rain and snow on Tuesday, Oyu struggled to make an impact on the leaderboard when conditions improved on Wednesday, ending up with the 18th-fastest time.
He was also forced to sit out the final part of the afternoon session, when the fastest times were set, owing to a mechanical issue that forced him to stop out on-track, albeit not causing a red flag.
Despite that, he still finished one place ahead of Nakajima teammate Naoki Yamamoto in the final order.
While satisfied to have improved his long run pace since the preceding test at Suzuka, Oyu admitted that his qualifying speed appears to have deserted him just weeks before next month's Fuji season opener.
"Regarding the long run, which was a weak point before, I was able to test that and there was no problem," Oyu told Motorsport.com's Japanese edition. "But as the results show, I think one-lap speed is our weakness.
"The feeling honestly was not good. It didn’t feel as if I was using new tyres. But there was a good thing in the sense that this is a weakness we didn’t experience before, so it was at least positive to make this kind of new discovery.
"I think we can expect to have good race pace. There isn’t much [running time] before the opening round, but I want to work on my one-lap speed."
Oyu topped the Fuji pre-season test last year with a best time of 1m21.371s, meaning his time on Wednesday was some six tenths slower.
He also went on to qualify second on the grid for the Fuji season opener behind eventual champion Tomoki Nojiri.
Asked why he felt the situation had changed from 12 months ago, Oyu replied: "Since the [rear] tyres changed, it could be that some things are not gelling well, like the feeling in cold conditions. But I think there’s no issue if it gets hotter."
Oyu also admitted that the Suzuka test earlier in the month had been a struggle, explaining that he was suffering from understeer.
"It could be the case [that we didn’t find the right set-up], but I think our weak points from before, mainly understeer, were more noticeable," said Oyu, who was ninth-fastest on the opening day at Suzuka before improving to second behind pacesetter Sho Tsuboi on the second day.
