Grinding SM420 with 80w90 when warm. Heavier oil needed?

[jumpsoffrock]

As stated in the title my 1967 C50 4speed shifts amazing right now when it's about 50ºF outside with the 80w90 oil, brand new.

But when the weather was warm and/or the trans starts to get warm, it'll start to want to grind unless I take an eon to shift and slide it in like a feather--even when double clutching which I always do.


Am I a perfect candidate for a heavier oil? I was also thinking about adding some Lucas oil "stabilizer"(it's just a thickener).

Any advice appreciated, thank you.