got a 380 in manual 180,000kms, i was using castrol vmx 75-85...worked a charm for a long time......oil changed every 10,000kms.....then after ticking up to 120,000kms it started getting crunchy and notchy.....so switch over to tried synthetic oils, i tried

redline mtl
nulon 75-90
amsoil 75-90

worked great but now climbing up to 180,000kms it has started to be notchy and grindy again.....regardless of new/old oil.....then a put some castrol vmx 75-85 with nulon g70.....and it is still the same...

is it bad switching mineral and synthetic then back to mineral?

any suggestions on oil?

Redline MTL are thinner and should improve your shifting compare to 75w90 viscosity but since you tried all, i would suggest you try Amsoil Synchromesh MTF or any Synchromesh type fluid, synchromesh fluid might solve your problem.

If Redline does not work then you have issues with your box

My 380 box in my Magna has done over 300k and is as smooth as silk