Davie,
I cannot recollect that particular thread. What vehicle do you require the 30 HD for? Some of the older engines were specified to use SAE 30 engine oil, which was a non-detergent, if this has been in use and the engine not stripped and cleaned, the use of modern type of high detergency SAE 30 as used in diesel engines, will scour the sludge and can then block oil ways, etc.
The non-detergent SAE 30 straight engine oil ( or very low detergent) is available from makers like Morris Lubricants, Comma, Silkolene, etc. for classic and vintage engines.