The synthetic oils allow for greater intervals than the standard oils we used to use. Synthetic doesn't break down as easily, so it can be used longer. 7500 miles is a good interval, but that depends on your daily use. Short trips where the engine oil does not get hot does more harm than anything else. Hot oil will cause water to boil off which was introduced by normal condensation from cooling oil. What water doesn't burn off will turn into acid which eats at various components.

Both of your cars can use the same viscosity, just not the same oil filter (darn). And I wouldn't switch viscosities between winter and summer. Check the back of the owner's manual for table of ambient temps and oil weight required. Use the XC90s manual, not the V70. Oils have become much better since 1998.
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My name is Klaus and I am a V ♂ lv ♂ holic.