Headstock clean-up

[Rickabilly]

Diesel engine oil (as opposed to Diesel fuel oil) contains detergent additives specifically designed to remove oil sludge. There is a chance that the sludge was formed by some coolant leaking into the headstock, might be worth making sure no future ingress is possible. You will need to fill with the oil and use for awhile given that the sludge won't hurt anything leave it for a month and use it regularly to get it up to temperature at least a few times, even run with no load just to keep the oil swishing about.
Regards
Rick