the concensus is to change the hydraulic tensioner at the second t-belt change, that is 140k miles.

Remember that if your s70 is a late '98 model and has the mechanical tensioner, the t-belt change is at 105k miles and mechanical tensioner is replaced at the same time.

My V70 is a late '98 model with the mechanical tensioner, the dealer checked the belt at 78k miles and it looked like new, so they rec going to 105k miles. I plan to replace mine at about 95k miles with the mechanical tensioner, that would be about 2 years from now since I average about 8k miles per year.
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'88 240, '92 745, '98 v70 John, Tampa Bay