I've used the Shell Helix and "Quantum" on my 99 T5 which is Castrol and had no problems or ill effects with either oil.
I usually put 5/40 fully synth in but as I'm not paying for it, the cost doesn't matter to me.
Engine is quiet on start up with this stuff but it does consume a bit if I'm honest.
If you change the oil often enough though you could happily get away with 10/40
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