Hi

Go for a well known brand that you can trust............when your Calibra was new and all engine tolerances were set to precision then you would have been advised to follow the manufacture recommendation .

This was most likely fully synthetic ....5W/30 grade for a turbo engine however......as the car gets older and engine tolerances start to increase

Then I use and recommend....Semi Synthetic..10W/ 40 grade Mobil Super 2000 X1.

Sorry this is long winded but it puts into perspective my reasons......Thin oil would find it's way to leak past oil seals.....gaskets .....piston rings

valve seals and so on.  This has been my choice and my car has been running fine and sweet.

One final note I change my oil twice a year to be safe especially when factoring in the health of the Turbo.