I bought my '83 in May with 213K on it and have 220K now. I used Castrol 20/50 for the first 2 oil changes until I realized we have Del-Vac 15/40 here at work so I used that this time around. I'm a die-hard synthetic user and I'm wonering if at this stage in life,it's worth it to use something like Mobil 1 15/50. Any feedback would be appreciated... TIA..