The 2012 DTM champion experienced a rollercoaster first race in the Japanese series sharing the Team Studie BMW M4 GT3 he shared with Seiji Ara, the pair finishing in sixth after Spengler qualified an impressive third.
Spengler took the start and initially held position, but had already been passed for third by the #88 JLOC Lamborghini when he was turned around by the Goodsmile Mercedes of Tatsuya Kataoka, who earned a drive-through penalty.
Further time was lost in a crowded pitlane when Spengler was among a bunch of cars to stop for wet tyres on lap 18, but the pace advantage afforded by his Michelin tyres allowed him to make up the lost ground.
After handing off to Ara on lap 39, an unlikely podium looked to be in the offing before the weather took a fresh turn for the worse, the Studie crew losing out again amid a late dash for wet tyres before the race was red-flagged and halted with 59 laps completed by the GT300 runners.
"In the dry at the start of the race, the pace was quite good," Spengler told Motorsport.com. "But when it started raining we started to struggle on slicks a bit more than some others, and then I got spun around by the [Goodsmile] Mercedes and we lost a lot of positions.
"Our pit position was also a bit unlucky. When we had a large group of cars pitting together, we could not ‘park’ properly. After the first stop I think we were down to about P17 or P18 because we were stuck.
"But the team did a great decision with the tyres. To begin with I struggled to get the tyres up to temperature and I fell down to P21, but I came all the way back to P6. The tyres were working well, the car was quick and we made up a lot of time - I think at one stage I was four seconds quicker than anyone else!
"Then I gave the car to Ara-san, and he managed to get as high as P3. But after the first red flag we were again punished by our pit position, we got stuck again and we lost another three positions.
"Then there was no more green running, so that was it. We were quite unlucky at certain points, but considering the conditions, I think we can accept sixth place. It could have been much worse.
"I would have loved to come away with a podium from my first SUPER GT race, I’m a bit disappointed there, but it’s good points for the team."
Spengler now takes a step away from SUPER GT as he turns his focus to his Italian GT campaign with BMW, before he returns for the final four races of the season.
"I knew the weather can do some interesting things in Japan, just from watching Formula 1 races in Japan from the past, but I wasn’t expecting something like this!" Spengler smiled. "Anyway, I enjoyed my first race weekend here a lot.
"It’s an honour to race in SUPER GT with BMW and Team Studie, they are great people and I like the way they work. I can’t wait to come back at Suzuka in August and do the rest of the season."
Two-time GT500 champion Masataka Yanagida will take Spengler's place alongside Ara in the Studie BMW for the next three races.
Join Motorsport.tv to watch the full SUPER GT Okayama race on replay here.
