Originally Posted by OceanRuns
Picked up a 1985 Dodge 600 convertible, 96k miles, engine is the Chrysler 2.2 turbo.
Previous owner changed the oil shortly before selling it and used 5w-20 (presumably synthetic). Per his notes previous oil changes were 5w-30. He told him he didn't have the 30w on hand, so just used what he used for one of his modern cars.
So, should I change it or run the 5w-20?
Come on, a 2.2 L engine putting out only 146 hp and 168 lbâ‹…ft? You would be even fine with a 0W-16. There is absolutely no need to drain the 5W-20.
Any modern oil is fine for this 1980s engine.
Back in the 1980s, they didn't have the HTHS requirement, which means 5W-30 oils of the 1980s were equivalent to the modern 5W-20 oils, with HTHS ~ 2.7 cP. That's why in the 1980s you would see 10W-30 being recommended but not 5W-30, as 5W-30 had lower HTHS than 10W-30 back then, not because of permanent oil shear but by design in those years. The table below shows the HTHS values for new, fresh oils of the 1980s era. The paper I linked also shows that for the used oils, KV shears but the HTHS remains constant because the oil thickening due to oxidation compensates for the permanent oil shear and makes up for the small drop in the HTHS due to oil shear with mileage.
Source:
Investigations of engine-oil shear stability in laboratory tests and road tests
HTHS limits were not introduced until February 1992. They were not applied to the non-W part of the SAE viscosity until March 1993. 0W-40/5W-40/10W-40 oils still had a 2.9 cP lower limit (same as for SAE 30) until November 2007.
Source:
Extending SAE J300 to viscosity grades below SAE 20
In conclusion:
- Any modern oil is fine for this engine, perhaps with the exception of SAE 0W-16.
- 5W-30 oils of the 1980s were equivalent to 5W-20 oils of today because the HTHS limits had not yet been introduced.
- If there is oil consumption due to bad valve-stem oil seals, a 15W-40 HDEO would help. A 15W-40 HDEO is also recommended for flat tappets because of its very thick base oil.