Olivia Newton-John and John Travolta were hopelessly devoted to each other for decades

Olivia Newton-John and John Travolta were a couple only when they appeared together on the big screen. But their platonic love for each other endured long after they played Sandy and Danny in the hit 1978 movie “Grease.”

“My dearest Olivia, you made all of our lives so much better,” Travolta, 68, wrote Monday on Instagram after learning of Newton-John’s death that day at age 73. “Your impact was incredible. I love you so much. We will see you down the road and we will all be together again. Yours from the first moment I saw you and forever! Your Danny, your John!”

Theirs was a friendship forged in the extreme heat of “Grease,” filmed when they were both in their 20s. In 2018, Travolta explained the strength of their decades-long relationship.

“When you share that kind of meteoric success — and nothing has been able to exceed it — you share a bond,” he told People as the movie marked its 40th anniversary. “I’ve been through her having a child, getting divorced, losing her sister. She’s been through my getting married, having children. It’s wonderful and full of shared memories.”

Newton-John gave her take on their relationship as well, telling People, “We did something life-changing, making that film. [At the premiere] you got the feeling from the energy that something was happening. It was a huge response. I feel grateful to be a part of that and to have worked with him. We’ve stayed friends ever since.”

Travolta always took the time to reach out to her when things were tough, she said.

Paul Hogan, left, with Olivia Newton-John and John Travolta at the 2013 G’Day USA black-tie gala in L.A.

(Getty Images for G’Day Australia)

Over the decades, the actor-singers could be seen mingling at annual G’Day USA events promoting diplomatic and economic ties between the United States and Newton-John’s home country, Australia. Travolta, a pilot who has long been referred to as an “honorary Australian,” also has a long-standing relationship with event sponsor Qantas.

At the organization’s 2013 black-tie gala, Newton-John helped present “Australian icon” honors to “‘Crocodile’ Dundee” star Paul Hogan. Travolta was named a Goodwill Ambassador for Australia that same year — a few days after singing with Keith Urban at a cocktail party kicking off G’Day USA’s week of events.

Olivia Newton-John and John Travolta attend a “Grease” 40th-anniversary screening in Beverly Hills in August 2018.

(Alberto E. Rodriguez / Getty Images)

Travolta and Newton-John were also excellent “Grease” ambassadors in the more than 40 years since the movie premiered, showing up when Paramount Pictures released a half-dozen of its top musicals on DVD and participating in events like the 2019 “Meet ’n’ Grease” session that followed a movie sing-along in Palm Beach, Fla. “We love singing together,” Travolta said during that Q&A. “So anything is possible.”

At a 40th-anniversary celebration of the movie, speaking on a panel discussion presented by the motion picture academy, he said Newton-John was his “favorite thing about doing ‘Grease.’” He remembered giving input when she was cast: “I said there is no other person in the universe other than Olivia Newton-John to play Sandy. You must cast her.”

Newton-John wasn’t so confident. She had been in a musical before and it hadn’t gone well, she revealed at the event, plus she was nervous about playing a teenager when she was in her late 20s.

“Then they sent John to meet me,” she said. “And who can say no, right? This gorgeous guy walks up my driveway, [with] these bright blue eyes, and he was so sweet and melted me.”

Alas, with the good must come the bad. There were two products of the Travolta/Newton-John friendship that might have been better left on the shelf.

The first, “Two of a Kind,” was a box-office flop released in 1983. While “Grease” made almost $400 million worldwide, the subsequent high-concept comedy pulled in a mere $24 million. A trailer for the movie even tried to sell the singing talents of the “Grease” stars, promising “lots of good music from the stars themselves, as well as Boz Skaggs and Chicago.”

“Give me a break,” movie critic Roger Ebert wrote in his “Two of a Kind” review. “Don’t send me any more movies where four angels in heaven ask God to give mankind a second chance, and God agrees — on the condition that John Travolta reforms. This movie should have been struck by a lightning bolt.”

The other clunker was “I Think You Might Like It,” a cheesy country-pop tune about gathering back home for Christmas, found on Travolta and Newton-John’s 2012 album, “This Christmas.” It was the lone original song on an album full of holiday standards.

The best way to experience the forgettable ditty is to dive in headfirst. Just click play on the incredibly hokey music video above and remember — good friends stick together through thick and thin. Especially when those friends are Olivia Newton-John and John Travolta.