This is about the easiest solution I’ve come across. Fits in the oil pan w/o a bung or adapter. Plenty of probe length so temp reading not too distorted by oil temp. Just have to take care to get the right gauge. VDO makes two types of gauge calibrations, US and “Other”. I’m not sure which this is.
http://www.bmpdesign.com/product-exec/product_id/864/category_id/27/search_year/1988/search_model/32