My car has sat through the Winter in an unheated garage. When I pulled the valve cover the driver side rear set of rockers are milky, none of the others are and the inside of the valve cover on that end has a milky substance. Is this because of setting all Winter or do I have an issue?

Last time I saw that it was a bad head gasket.

I would highly suspect it’s just condensation. Wipe it clean, put back together and run it. Watch the dipstick, but I bet it’s fine.

Two things you can do to fix that, drive it long enough2 t 0 4 hour trip to get the oil temps above 160F or use something to heat the oil up in the motor or drain it and heat it up on a stove, If you heat it up on a stove be very careful that you don't have the condensation settled to the bottom of the container and then create a lot of turbulence when it turns to steam from the bottom of the container

I've only seen that on one of my engines: it was fresh off the dyno, no winter or cold weather, and on both sides in the heads & valve covers.
That engine had a litany of problems, which I doubt yours has, but head gasket sealing was a possible cause.
One unpleasant discovery: the builder - Bob Lambeck - had actually milled through the valve reliefs in the pistons, about 1/4" x 1/16".

None of my daily cars that have sat outside in Idaho winters have had that condensation, and the other cars never see less than 44 degrees in the shop resting over winter.

I agree. Run it for a bit and see if it improves.

That is A LOT of condensation. I have had several cars sit in unheated garages through -10 or below outside temps and have never seen condensation anywhere, especially on the dipstick. One can change the oil and cross your fingers but I wouldn't place any bets on it.
not knowing your conditions you might want to google what causes condensation and see if any of those apply. IE: warm humid air to a cold surface. Think a cold beer on a humid day. Good luck and keep us posted

condensation , time for a long summer drive !