PB Blaster is good stuff
It's become automatic to do a penetrating squirt with crusty nuts.
I use it on just about everything, but some fasteners like exhaust get the ultimate penetrant;
Ed's Red Bore Cleaner
1 part Dexron II, IIe or III ATF, GM Spec. D-20265 or later.
1 part Kerosene - deodorized, K1
1 part Aliphatic Mineral Spirits, Fed. Spec. TT-T-2981F, CAS #64741-49-9, or substitute "Stoddard Solvent", CAS #8052-41-3, or equivalent, (aka "Varsol")
1 part Acetone, CAS #67-64-1.