All sorted now Shell Helix recing fluid 75w 90 Mail order from Deamon Tweaks next day. Transmission out for clutch replacement. It now cant handle the 500bhp remap so sachs competition unit being fitted today together with slave cylinder and accumulator.
Thought it would be a good time to change the oil.
Not to put too fine a point on it but with the selection of Shell Helix you have stepped off the reservation regarding Porsche approved tranny/diff fluids.
The problem is the Helix fluid may be incompatible with the transmission and its lubrication needs.
However, given that the engine has been tweaked to up its HP output there is no guarantee the Porsche approved fluids would have sufficed.
But with that much extra HP flowing through the transmission it may not matter that much.
The transmission because it has sufficient margin it can tolerate the higher output of the engine and because the Shell Helix transmission fluid does all the right things (and does none of the bad things) the transmission may be just fine.
Or the transmission doesn't have sufficient margin in which case the fluid choice may not matter that much.
No fluid is going to make an input shaft stronger, tougher.
BTW, remember the front diff can receive up to to 40% of the engine's torque so the front diff can receive more torque than its design was intended to handle.
If you believe Shell Helix will help the tranny/rear diff survive this added torque load you have to believe it can only help the front diff.
So, you might consider treating the front diff to a filling of Shell Helix fluid as well.