Have a 1987 Chevy v20 with the 6.2L Diesel and 4WD and manual 4-speed transmission. Recently had the transmission fluid flushed and re-filled. Ever since it has been shifting very roughly. Mechanic used 75W-90 Full Synthetic. Indicated it would take awhile to break in but curious if that’s the right fluid for this transmission? Also, noticed that the fluid in the hydraulic clutch reservoir is quite brown so thinking I need to flush that and put new DOT3 in as well.

Anyone know what is best transmission fluid to use on this truck? I seem to find everything from AT fluid like Dexron II or III but also have seen full synthetic 75-90 suggested.

Welcome any insight.

Thanks